Benefit Plans - Information Regarding Defined Benefit Pension Plans and Other Post Retirement Benefit Plan (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Pension Benefits
Change in benefit obligation:
Benefit obligation at beginning of year	$ 270,272	$ 230,361
Service cost	0	0	$ 0
Interest cost	9,929	10,450	9,549
Actuarial (gain) loss	(16,739)	48,633
Benefits paid	(11,111)	(19,172)
Benefit obligation at end of year	252,351	270,272	230,361
Change in plan assets:
Fair value of plan assets at beginning of year	187,074	189,367
Actual return on plan assets	3,881	11,264
Employer contribution	543	5,615
Benefits paid	(11,111)	(19,172)
Fair value of plan assets at end of year	180,387	187,074	189,367
Funded status at end of year	(71,964)	(83,198)
Other Postretirement Benefits
Change in benefit obligation:
Benefit obligation at beginning of year	28,545	18,766
Service cost	2,129	1,241	1,578
Interest cost	1,004	930	1,279
Actuarial (gain) loss	(6,179)	8,371
Benefits paid	(554)	(763)
Benefit obligation at end of year	24,945	28,545	18,766
Change in plan assets:
Fair value of plan assets at beginning of year	0	0
Actual return on plan assets	0	0
Employer contribution	554	763
Benefits paid	(554)	(763)
Fair value of plan assets at end of year	0	0	$ 0
Funded status at end of year	$ (24,945)	$ (28,545)