Benefit Plans Benefit Plans - Allocation of Plan Assets (Additional Information) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Private equity funds
Defined Benefit Plan Disclosure [Line Items]
Entities that calculate net asset value per share, unfunded commitments	5,529	5,529
Commercial Real Estate [Member] | Real estate common collective trusts | Common collective trusts [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan asset underlying investment portfolio holdings percentage	44.00%	45.00%
Residential Real Estate [Member] | Real estate common collective trusts | Common collective trusts [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan asset underlying investment portfolio holdings percentage	21.00%	23.00%
Retail Site [Member] | Real estate common collective trusts | Common collective trusts [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan asset underlying investment portfolio holdings percentage	24.00%	18.00%
Industrial, Cash and Other [Member] | Real estate common collective trusts | Common collective trusts [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan asset underlying investment portfolio holdings percentage	11.00%	14.00%
Fixed income securities | Other | Common collective trusts [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan asset underlying investment portfolio holdings percentage	58.00%	54.00%
Collateralized Mortgage Backed Securities [Member] | Other | Common collective trusts [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan asset underlying investment portfolio holdings percentage	27.00%	29.00%
Corporate Bond Securities [Member] | Other | Common collective trusts [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan asset underlying investment portfolio holdings percentage	47.00%	42.00%
U.S. Treasury and Other [Member] | Other | Common collective trusts [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan asset underlying investment portfolio holdings percentage	26.00%	29.00%
Russell 1000 Fund Large Cap Index Funds [Member] | Other | Common collective trusts [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan asset underlying investment portfolio holdings percentage	42.00%	46.00%
Plan asset underlying investment redemption Notice	1 day
Long Short Equity [Member] | Multi-strategy hedge funds
Defined Benefit Plan Disclosure [Line Items]
Plan asset underlying investment portfolio holdings percentage	44.00%	35.00%
Arbitrage and Event Investments [Member] | Multi-strategy hedge funds
Defined Benefit Plan Disclosure [Line Items]
Plan asset underlying investment portfolio holdings percentage	44.00%	45.00%
Directional Trading, Fixed Income and Other Investments [Member] | Multi-strategy hedge funds
Defined Benefit Plan Disclosure [Line Items]
Plan asset underlying investment portfolio holdings percentage	10.00%	20.00%
Buyout Private Equity Funds [Member] | Private equity funds
Defined Benefit Plan Disclosure [Line Items]
Plan asset underlying investment portfolio holdings percentage	50.00%	50.00%
Special Situation Private Equity and Debt Funds | Private equity funds
Defined Benefit Plan Disclosure [Line Items]
Plan asset underlying investment portfolio holdings percentage	25.00%	25.00%
Venture Private Equity Funds [Member] | Private equity funds
Defined Benefit Plan Disclosure [Line Items]
Plan asset underlying investment portfolio holdings percentage	25.00%	25.00%
Real estate common collective trusts
Defined Benefit Plan Disclosure [Line Items]
Plan assets redemptions per period	0.25
Plan asset underlying investment redemption Notice	45 days
Multi-strategy hedge funds
Defined Benefit Plan Disclosure [Line Items]
Plan assets redemptions per period	0.5
Plan asset underlying investment redemption Notice	95 days
Arbitrage and Event Investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan asset underlying investment portfolio holdings percentage	45.00%	40.00%
Directional Trading, Fixed Income and Other Investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan asset underlying investment portfolio holdings percentage	20.00%	25.00%