Benefit Plans (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Pension benefits
Change in benefit obligation:
Benefit obligation at beginning of year	$ 230,361	$ 260,108
Service cost	0	0	2,025
Interest cost	10,450	9,549	10,992
Actuarial loss (gain)	48,633	(28,749)
Benefits paid	(19,172)	(10,547)
Benefit obligation at end of year	270,272	230,361	260,108
Change in plan assets:
Fair value of plan assets at beginning of year	189,367	160,683
Actual return on plan assets	11,264	33,583
Employer contribution	5,615	5,648
Benefits paid	(19,172)	(10,547)
Fair value of plan assets at end of year	187,074	189,367	160,683
Funded status at end of year	(83,198)	(40,994)
Other Postretirement Benefits
Change in benefit obligation:
Benefit obligation at beginning of year	18,766	35,476
Service cost	1,241	1,578	1,061
Interest cost	930	1,279	1,378
Actuarial loss (gain)	8,371	(18,572)
Benefits paid	(763)	(995)
Benefit obligation at end of year	28,545	18,766	35,476
Change in plan assets:
Fair value of plan assets at beginning of year	0	0
Actual return on plan assets	0	0
Employer contribution	763	995
Benefits paid	(763)	(995)
Fair value of plan assets at end of year	0	0	0
Funded status at end of year	$ (28,545)	$ (18,766)