Accumulated Other Comprehensive Income (Loss) - Income Tax Expense (Benefit) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Defined Benefit Pension Plan Adjustments
Accumulated Other Comprehensive Income (Loss)
Income tax benefit included in accumulated other comprehensive income (loss)	$ 15,800		$ 15,800		$ 17,100
Components of income tax expense (benefit) included in other comprehensive income (loss)
Income tax expense (benefit) included in other comprehensive income (loss)	586	$ 246	1,185	$ 733
Derivatives
Accumulated Other Comprehensive Income (Loss)
Income tax benefit included in accumulated other comprehensive income (loss)	400		400		$ 100
Components of income tax expense (benefit) included in other comprehensive income (loss)
Income tax expense (benefit) included in other comprehensive income (loss)	$ 42	$ (282)	$ (271)	$ (324)