Accumulated Other Comprehensive Income Amounts Reclassified (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended		12 Months Ended
		Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Accumulated Other Comprehensive Income (Loss) [Line Items]
Prior service benefit		$ 1,662	$ 2,070	$ 3,325	$ 3,529	$ 7,455	$ 6,289
Interest on long-term debt and revolving credit facility		(14,197)	(11,647)	(26,933)	(23,469)	(50,282)	(47,422)
Income tax expense		9,505	14,763	10,489	16,236	35,341	40,647
Net income (loss)		21,007	29,993	24,459	34,596	80,994	86,194
Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net income (loss)		(505)	174	2,194	2,673	6,200	1,397
Reclassification out of Accumulated Other Comprehensive Income | Accumulated Defined Benefit Plans Adjustment
Accumulated Other Comprehensive Income (Loss) [Line Items]
Prior service benefit	[1]	1,662	2,070	3,325	3,529	7,455	6,289
Net loss	[1]	(2,250)	(1,829)	(4,500)	(2,952)	(7,730)	(8,004)
Income (Loss) before income tax	[1]	(588)	241	(1,175)	577	(275)	(1,715)
Income tax expense		291	(90)	622	(221)	292	574
Net income (loss)	[1]	(297)	151	(553)	356	17	(1,141)
Reclassification out of Accumulated Other Comprehensive Income | Accumulated Net Unrealized Investment Gain (Loss)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net realized gain (loss) on sale of securities		(182)	102	3,563	2,967	7,946	3,432
Income (Loss) before income tax		(182)	102	3,563	2,967	7,946	3,432
Income tax expense		47	(10)	(701)	(566)	(1,508)	(679)
Net income (loss)		(135)	92	2,862	2,401	6,438	2,753
Reclassification out of Accumulated Other Comprehensive Income | Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss) [Line Items]
Interest on long-term debt and revolving credit facility		(116)	(109)	(230)	(216)	(452)	(425)
Income (Loss) before income tax		(116)	(109)	(230)	(216)	(452)	(425)
Income tax expense		43	40	115	132	197	210
Net income (loss)		$ (73)	$ (69)	$ (115)	$ (84)	$ (255)	$ (215)

[1] These items are included in the computation of net periodic benefit cost. See Note I, Employee Benefits, for additional information.