Accumulated Other Comprehensive Income (Summary of Changes in Accumulated Other Comprehensive Income) (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2021	Mar. 31, 2020
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at the beginning of the period		$ 3,715,023	$ 3,332,988
Other comprehensive income (loss) before reclassifications		3,383	(32,051)
(Gain) loss reclassified from accumulated other comprehensive loss to net income		2,973	1,150
Share of investee’s other comprehensive income, net of tax of $346	[1]	1,217	0
Net current-period other comprehensive loss		7,573	(30,901)
Balance at the end of the period		3,661,267	3,772,598
Share of investee's other comprehensive income, tax expense		346
AOCI Attributable to Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at the beginning of the period		(38,590)	(29,372)
Share of investee’s other comprehensive income, net of tax of $346		1,217
Balance at the end of the period		$ (31,017)	$ (60,273)

[1] The amount for 2021 is shown net of deferred tax expense of $0.3 million.