Accumulated Other Comprehensive Income (Loss) - Reclassifications (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Reclassifications out of other comprehensive income (loss)
Amortization of loss and settlement loss									$ 118,420	$ 123,798	$ 115,974
Income tax provision on comprehensive income									591	1,357	5,910
Net income	$ 214	$ 279	$ (1,445)	$ (154)	$ 435	$ (105)	$ (455)	$ (975)	(1,106)	(1,100)	(8,099)
Retirement plans | Reclassifications out of other comprehensive income (loss)
Reclassifications out of other comprehensive income (loss)
Amortization of loss and settlement loss									2,424	8,862	195
Income tax provision on comprehensive income									921	3,368	74
Net income									$ 1,503	$ 5,494	$ 121