Accumulated Other Comprehensive Income - Changes in AOCI (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Changes in AOCI by component
Beginning Balance	$ 4,851,700	$ 4,864,796	$ 4,862,576	$ 5,137,014
Ending Balance	4,847,112	4,885,693	4,847,112	4,885,693
Cumulative Unrealized Gain (Loss) on Available-for- Sale Securities
Changes in AOCI by component
Beginning Balance	41,310	42,930	44,057	50,996
OCI before reclassifications	(822)	(581)	(3,569)	(8,647)
Amounts reclassified from AOCI	(2)	0	(2)	0
Net period OCI	(824)	(581)	(3,571)	(8,647)
Ending Balance	40,486	42,349	40,486	42,349
Foreign Currency Translation
Changes in AOCI by component
Beginning Balance	0	(64)	(64)	(64)
OCI before reclassifications	0	0	0	0
Amounts reclassified from AOCI	0	0	64	0
Net period OCI	0	0	64	0
Ending Balance	0	(64)	0	(64)
Accumulated Other Comprehensive Income
Changes in AOCI by component
Beginning Balance	41,310	42,866	43,993	50,932
OCI before reclassifications	(822)	(581)	(3,569)	(8,647)
Amounts reclassified from AOCI	(2)	0	62	0
Net period OCI	(824)	(581)	(3,507)	(8,647)
Ending Balance	$ 40,486	$ 42,285	$ 40,486	$ 42,285