Shareholders' Equity - AOCI Roll Forward (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning Balance	$ 2,564,371	$ 2,450,941	$ 2,608,434	$ 2,345,626
Other comprehensive (loss)/income	(4,563)	22,734	5,580	30,250
Ending Balance	2,617,063	2,522,807	2,617,063	2,522,807
Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning Balance			9,184
Other comprehensive income before reclassifications, net of tax			13,985
Reclassifications from accumulated other comprehensive loss, net of tax			(8,571)
Other comprehensive (loss)/income			5,414
Ending Balance	14,598		14,598
Defined Benefit and Retiree Healthcare Plans
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning Balance			(35,362)
Other comprehensive income before reclassifications, net of tax			0
Reclassifications from accumulated other comprehensive loss, net of tax			166
Other comprehensive (loss)/income			166
Ending Balance	(35,196)		(35,196)
Accumulated Other Comprehensive Loss
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning Balance	(16,035)	(55,648)	(26,178)	(63,164)
Other comprehensive income before reclassifications, net of tax			13,985
Reclassifications from accumulated other comprehensive loss, net of tax			(8,405)
Other comprehensive (loss)/income			5,580
Ending Balance	$ (20,598)	$ (32,914)	$ (20,598)	$ (32,914)