Benefit Plans - Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Amounts within accumulated other comprehensive loss that had not yet been recognized as components of net periodic benefit cost
Tax impact	$ (57,461)	$ (33,463)	$ (33,591)	$ (29,875)
Pension Benefits
Amounts within accumulated other comprehensive loss that had not yet been recognized as components of net periodic benefit cost
Net actuarial loss	51,034	45,579	59,811
Total, pretax effect	51,034	45,579
Tax impact	(20,158)	(18,001)
Ending balance in accumulated other comprehensive loss	30,876	27,578
Amounts within accumulated other comprehensive loss expected to be recognized as components of net periodic benefit cost in next year
Amortization of net actuarial loss	7,996
Total to be recognized in 2017	7,996
Other Benefits
Amounts within accumulated other comprehensive loss that had not yet been recognized as components of net periodic benefit cost
Net actuarial loss	282	619	$ 1,669
Prior service credit	(1,338)	(1,767)
Total, pretax effect	(1,056)	(1,148)
Tax impact	417	453
Ending balance in accumulated other comprehensive loss	(639)	$ (695)
Amounts within accumulated other comprehensive loss expected to be recognized as components of net periodic benefit cost in next year
Amortization of prior service credit	(429)
Total to be recognized in 2017	$ (429)