Pension and Other Post-retirement Benefit Plans (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Pension Benefits
Components of net periodic benefit cost:
Service cost	$ 184	$ 147	$ 552	$ 445
Interest cost	2,364	2,485	7,092	7,688
Expected return on plan assets	(3,291)	(3,094)	(9,873)	(9,496)
Amortization of:
Prior service cost (benefit)	78	78	232	232
Actuarial loss (gain)	940	619	2,821	1,902
Settlements		61		61
Net periodic cost (benefit)	275	296	824	832
Anticipated contributions			36,000
Contributions made			13,900
Contributions by employer			7,800
Pension Benefits | Scenario, Previously Reported
Amortization of:
Anticipated contributions			10,800
Other Post-Retirement Benefits
Components of net periodic benefit cost:
Service cost	34	46	177	157
Interest cost	123	99	525	784
Amortization of:
Prior service cost (benefit)	(461)	(372)	(1,325)	(1,115)
Actuarial loss (gain)	(343)	(479)	(581)	(570)
Net periodic cost (benefit)	(647)	$ (706)	(1,204)	$ (744)
Contributions made			800
Expected contributions	$ 1,100		1,100
Pension Benefit Guaranty Corporation | Pension Benefits
Amortization of:
Contributions by employer			6,400
Pension Benefit Guaranty Corporation | Pension Benefits | Scenario, Previously Reported
Amortization of:
Anticipated contributions			$ 8,500