Retirement Benefits - Plan Asset Allocation (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Qualified Pension Benefits
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	626,173		$ 615,721		$ 531,183
Other Benefits
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	8,360		8,774		7,541
SERP Pension Benefits
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	0		0		0
Domestic large cap equity
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Target Allocation, Minimum	25.00%
Target Allocation	31.00%
Target Allocation, Maximum	40.00%
Domestic small cap equity
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Target Allocation, Minimum	0.00%
Target Allocation	9.00%
Target Allocation, Maximum	15.00%
Non-U.S. equity
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Target Allocation, Minimum	10.00%
Target Allocation	25.00%
Target Allocation, Maximum	30.00%
Fixed income
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Target Allocation, Minimum	15.00%
Target Allocation	25.00%
Target Allocation, Maximum	30.00%
Real estate
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Target Allocation, Minimum	0.00%
Target Allocation	0.00%
Target Allocation, Maximum	10.00%
Absolute return
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Target Allocation, Minimum	5.00%
Target Allocation	10.00%
Target Allocation, Maximum	15.00%
Cash and cash equivalents
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Target Allocation, Minimum	0.00%
Target Allocation	0.00%
Target Allocation, Maximum	5.00%
Fair Value, Measurements, Recurring | Qualified Pension Benefits | Level 3
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	65,251		62,278		55,614
Fair Value, Measurements, Recurring | Qualified Pension Benefits | Total
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	626,173		615,721
Fair Value, Measurements, Recurring | Other Benefits | Level 1
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	8,360		8,703
Fair Value, Measurements, Recurring | Other Benefits | Level 2
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	0		71
Fair Value, Measurements, Recurring | Other Benefits | Total
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	8,360		8,774
Fair Value, Measurements, Recurring | Subtotal | Qualified Pension Benefits | Level 1
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	350,429		431,936
Fair Value, Measurements, Recurring | Subtotal | Qualified Pension Benefits | Level 2
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	209,649		121,721
Fair Value, Measurements, Recurring | Subtotal | Qualified Pension Benefits | Level 3
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	65,251		62,278
Fair Value, Measurements, Recurring | Subtotal | Qualified Pension Benefits | Total
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	625,329		615,935
Fair Value, Measurements, Recurring | Equities: | Qualified Pension Benefits | Level 1
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	265,448		296,929
Fair Value, Measurements, Recurring | Equities: | Qualified Pension Benefits | Level 2
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	142,467		114,667
Fair Value, Measurements, Recurring | Equities: | Qualified Pension Benefits | Level 3
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	0		0
Fair Value, Measurements, Recurring | Equities: | Qualified Pension Benefits | Total
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	407,915		411,596
Fair Value, Measurements, Recurring | Domestic large cap equity | Qualified Pension Benefits | Level 1
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	134,765	[1]	157,874	[1]
Fair Value, Measurements, Recurring | Domestic large cap equity | Qualified Pension Benefits | Level 2
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	68,336	[1]	35,851	[1]
Fair Value, Measurements, Recurring | Domestic large cap equity | Qualified Pension Benefits | Level 3
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	0	[1]	0	[1]
Fair Value, Measurements, Recurring | Domestic large cap equity | Qualified Pension Benefits | Total
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	203,101	[1]	193,725	[1]
Fair Value, Measurements, Recurring | Domestic small cap equity | Qualified Pension Benefits | Level 1
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	59,657	[2]	62,867	[2]
Fair Value, Measurements, Recurring | Domestic small cap equity | Qualified Pension Benefits | Level 2
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	0	[2]	0	[2]
Fair Value, Measurements, Recurring | Domestic small cap equity | Qualified Pension Benefits | Level 3
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	0	[2]	0	[2]
Fair Value, Measurements, Recurring | Domestic small cap equity | Qualified Pension Benefits | Total
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	59,657	[2]	62,867	[2]
Fair Value, Measurements, Recurring | Non-U.S. equity | Qualified Pension Benefits | Level 1
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	71,026	[3]	76,188	[3]
Fair Value, Measurements, Recurring | Non-U.S. equity | Qualified Pension Benefits | Level 2
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	74,131	[3]	78,816	[3]
Fair Value, Measurements, Recurring | Non-U.S. equity | Qualified Pension Benefits | Level 3
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	0	[3]	0	[3]
Fair Value, Measurements, Recurring | Non-U.S. equity | Qualified Pension Benefits | Total
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	145,157	[3]	155,004	[3]
Fair Value, Measurements, Recurring | Tactical asset allocation | Qualified Pension Benefits | Level 1
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	0	[4]	0	[4]
Fair Value, Measurements, Recurring | Tactical asset allocation | Qualified Pension Benefits | Level 2
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	0	[4]	0	[4]
Fair Value, Measurements, Recurring | Tactical asset allocation | Qualified Pension Benefits | Level 3
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	0	[4]	0	[4]
Fair Value, Measurements, Recurring | Tactical asset allocation | Qualified Pension Benefits | Total
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	0	[4]	0	[4]
Fair Value, Measurements, Recurring | Fixed income | Qualified Pension Benefits | Level 1
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	72,331	[5]	135,007	[5]
Fair Value, Measurements, Recurring | Fixed income | Qualified Pension Benefits | Level 2
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	67,182	[5]	0	[5]
Fair Value, Measurements, Recurring | Fixed income | Qualified Pension Benefits | Level 3
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	0	[5]	0	[5]
Fair Value, Measurements, Recurring | Fixed income | Qualified Pension Benefits | Total
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	139,513	[5]	135,007	[5]
Fair Value, Measurements, Recurring | Absolute return | Qualified Pension Benefits | Level 1
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	0	[6]	0	[6]
Fair Value, Measurements, Recurring | Absolute return | Qualified Pension Benefits | Level 2
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	0	[6]	0	[6]
Fair Value, Measurements, Recurring | Absolute return | Qualified Pension Benefits | Level 3
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	65,251	[6]	62,278	[6]
Fair Value, Measurements, Recurring | Absolute return | Qualified Pension Benefits | Total
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	65,251	[6]	62,278	[6]
Fair Value, Measurements, Recurring | Mutual Funds | Qualified Pension Benefits | Level 3
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	0		0		0
Fair Value, Measurements, Recurring | Mutual Funds | Other Benefits | Level 1
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	8,301	[7]	8,703	[7]
Fair Value, Measurements, Recurring | Mutual Funds | Other Benefits | Level 2
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	0	[7]	0	[7]
Fair Value, Measurements, Recurring | Mutual Funds | Other Benefits | Total
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	8,301	[7]	8,703	[7]
Fair Value, Measurements, Recurring | Cash and cash equivalents | Qualified Pension Benefits | Level 1
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	12,650	[8]	0	[8]
Fair Value, Measurements, Recurring | Cash and cash equivalents | Qualified Pension Benefits | Level 2
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	0	[8]	7,054	[8]
Fair Value, Measurements, Recurring | Cash and cash equivalents | Qualified Pension Benefits | Level 3
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	0	[8]	0	[8]
Fair Value, Measurements, Recurring | Cash and cash equivalents | Qualified Pension Benefits | Total
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	12,650	[8]	7,054	[8]
Fair Value, Measurements, Recurring | Cash and cash equivalents | Other Benefits | Level 1
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	59	[9]	0	[9]
Fair Value, Measurements, Recurring | Cash and cash equivalents | Other Benefits | Level 2
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	0	[9]	71	[9]
Fair Value, Measurements, Recurring | Cash and cash equivalents | Other Benefits | Total
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	59	[9]	71	[9]
Fair Value, Measurements, Recurring | Net (payable) receivable | Qualified Pension Benefits | Total
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	844		(417)
Fair Value, Measurements, Recurring | Accrued income | Qualified Pension Benefits | Total
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets	0		$ 203

[1] Domestic large cap equity investments are comprised of (1) common stock, and a (2) commingled fund. Investments in common stock are valued using quoted market prices multiplied by the number of shares owned as of December 31, 2014. The investment in the commingled fund is valued at the net asset value per share multiplied by the number of shares held as of December 31, 2014.

[2] Domestic small cap equity investments are comprised of (1) common stock and a (2) mutual fund. The investments in common stock are valued using quoted market prices multiplied by the number of shares owned as of December 31, 2014. The investment in the mutual fund is valued using quoted market prices multiplied by the number of shares owned as of December 31, 2014.

[3] Non – US Equity investments are comprised of a (1) mutual fund; and a (2) commingled fund. The investment in the mutual fund is valued using quoted market prices multiplied by the number of shares owned as of December 31, 2014. The investment in the commingled fund is valued at the net asset value per share multiplied by the number of shares held as of December 31, 2014.

[4] The tactical asset allocation investment is comprised of a commingled fund, which is valued at the net asset value per share multiplied by the number of shares held as of the measurement date.

[5] Fixed income securities consist of a mutual fund. The investment in the mutual fund is valued using quoted market prices multiplied by the number of shares owned as of December 31, 2014.

[6] As of December 31, 2014 absolute return investments consist of two partnerships. The partnerships are valued using the financial reports as of December 31, 2014. These investments are a Level 3 under ASC 820 because the significant valuation inputs are primarily internal to the partnerships with little third party involvement.

[7] This is a publicly traded balanced mutual fund. The fund seeks regular income, conservation of principal, and an opportunity for long-term growth of principal and income. The fair value is determined by taking the number of shares owned by the plan, and multiplying by the market price as of December 31, 2014.

[8] The investment consists of a money market fund, which is valued at the net asset value per share of $1.00 per unit as of December 31, 2014. The money market fund invests primarily in commercial paper, notes, repurchase agreements, and other evidences of indebtedness which are payable on demand or short-term in nature.

[9] This is a money market fund. The money market fund investments are valued at the net asset value per share of $1.00 per unit as of December 31, 2014. The money market fund invests primarily in commercial paper, notes, repurchase agreements, and other evidences of indebtedness which are payable on demand or short-term in nature.