Accumulated Other Comprehensive Income (Loss) (Details 2) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Reclassifications out of other comprehensive income (loss)
Amortization of (gain) loss and settlement loss									$ 123,798	$ 115,974	$ 114,875
Income tax provision									1,357	5,910	8,782
Net income	$ 435	$ (105)	$ (455)	$ (975)	$ (1,976)	$ (1,516)	$ (2,239)	$ (2,368)	(1,100)	(8,099)	(10,488)
Reclassifications out of accumulated other comprehensive income (loss)
Reclassifications out of other comprehensive income (loss)
Amortization of (gain) loss and settlement loss									8,862	195	984
Retirement plans
Reclassifications out of other comprehensive income (loss)
Income tax provision									3,368	74	374
Net income									$ 5,494	$ 121	$ 610