Benefit Plans - Information Regarding Defined Benefit Pension Plans and Other Postretirement Benefit Plan (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Pension Benefits
Change in benefit obligation:
Benefit obligation at beginning of year	$ 252,351	$ 270,272
Service cost	0	0	$ 0
Interest cost	10,042	9,929	10,450
Actuarial loss (gain)	1,423	(16,739)
Benefits paid	(11,225)	(11,111)
Benefit obligation at end of year	252,591	252,351	270,272
Change in plan assets:
Fair value of plan assets at beginning of year	180,387	187,074
Actual return on plan assets	13,179	3,881
Employer contribution	429	543
Benefits paid	(11,225)	(11,111)
Fair value of plan assets at end of year	182,770	180,387	187,074
Funded status at end of year	(69,821)	(71,964)
Other Postretirement Benefits
Change in benefit obligation:
Benefit obligation at beginning of year	24,945	28,545
Service cost	1,863	2,129	1,241
Interest cost	994	1,004	930
Actuarial loss (gain)	(1,760)	(6,179)
Benefits paid	(599)	(554)
Benefit obligation at end of year	25,443	24,945	28,545
Change in plan assets:
Fair value of plan assets at beginning of year	0	0
Actual return on plan assets	0	0
Employer contribution	599	554
Benefits paid	(599)	(554)
Fair value of plan assets at end of year	0	0	$ 0
Funded status at end of year	$ (25,443)	$ (24,945)