Retirement and Postretirement Plans - Changes in Plans' Benefit Obligations (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Other Pension Plan, Postretirement or Supplemental Plans [Member]
Change in benefit obligation:
Benefit obligation at beginning of year	$ 26,384	$ 24,747
Merger transactions	0	0
Service cost	61	81	$ 11
Interest cost	829	1,076
Participant contributions	103	180
Plan settlements	0	0
Actuarial loss (gain)	(3,600)	2,077
Benefit payments	(1,938)	(1,777)
Benefit obligation at end of year	21,839	26,384	24,747
Pension Plan [Member]
Change in benefit obligation:
Benefit obligation at beginning of year	621,823	488,229
Merger transactions	0	146,403
Service cost	0	170	170
Interest cost	22,581	22,324
Participant contributions	0	0
Plan settlements	0	(70,612)
Actuarial loss (gain)	(38,859)	61,484
Benefit payments	(41,911)	(26,175)
Benefit obligation at end of year	$ 563,634	$ 621,823	$ 488,229