Accumulated Other Comprehensive Income (Summary of Changes in Accumulated Other Comprehensive Income) (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at the beginning of the period		$ 3,661,267	$ 3,772,598	$ 3,715,023	$ 3,332,988
Other comprehensive income (loss) before reclassifications		(1,136)	(4,197)	2,247	(36,248)
(Gain) loss reclassified from accumulated other comprehensive loss to net income		3,041	3,359	6,014	4,509
Share of investee’s other comprehensive income	[1]	1,358	0	2,575	0
Net current-period other comprehensive income		3,263	(838)	10,836	(31,739)
Balance at the end of the period		3,671,903	3,739,703	3,671,903	3,739,703
Share of investee's other comprehensive income, tax expense (benefit)		386		732
AOCI Attributable to Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at the beginning of the period		(31,017)	(60,273)	(38,590)	(29,372)
Share of investee’s other comprehensive income		1,358		2,575
Balance at the end of the period		$ (27,754)	$ (61,111)	$ (27,754)	$ (61,111)

[1] The amount for 2021 is shown net of deferred tax expense of $0.4 million for the three months ended June 30, 2021 and $0.7 million for the six months ended June 30, 2021.