Shareholders' Equity - AOCI Roll Forward (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning Balance	$ 2,573,755	$ 2,608,434
Other comprehensive income	(15,992)	10,143
Ending Balance	2,537,797	2,564,371
Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning Balance	16,546
Other comprehensive loss before reclassifications, net of tax	(13,041)
Reclassifications from accumulated other comprehensive loss, net of tax	(6,293)
Other comprehensive income	(19,334)
Ending Balance	(2,788)
Defined Benefit and Retiree Healthcare Plans
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning Balance	(37,030)
Other comprehensive loss before reclassifications, net of tax	0
Reclassifications from accumulated other comprehensive loss, net of tax	3,342
Other comprehensive income	3,342
Ending Balance	(33,688)
Accumulated Other Comprehensive Loss
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning Balance	(20,484)	(26,178)
Other comprehensive loss before reclassifications, net of tax	(13,041)
Reclassifications from accumulated other comprehensive loss, net of tax	(2,951)
Other comprehensive income	(15,992)
Ending Balance	$ (36,476)	$ (16,035)