Accumulated Other Comprehensive Income (Loss) - Income Tax Expense (Benefit) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Defined Benefit Pension Plan Adjustments
Accumulated Other Comprehensive Income (Loss)
Income tax benefit included in accumulated other comprehensive loss	$ 17,200		$ 17,200		$ 17,700
Components of income tax expense (benefit) included in other comprehensive income (loss)
Income tax expense (benefit) included in other comprehensive income (loss)	307	$ 298	621	$ 599
Derivatives
Accumulated Other Comprehensive Income (Loss)
Income tax benefit included in accumulated other comprehensive loss	300		300
Income tax expense in accumulated other comprehensive income					$ 400
Components of income tax expense (benefit) included in other comprehensive income (loss)
Income tax expense (benefit) included in other comprehensive income (loss)	$ (441)	$ 112	$ (712)	$ (313)