Accumulated Other Comprehensive Income - Changes in AOCI (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Changes in AOCI by component
Beginning Balance	$ 4,854,785	$ 4,817,351	$ 4,862,576	$ 5,137,014
Ending Balance	4,851,700	4,864,796	4,851,700	4,864,796
Cumulative Unrealized Gain (Loss) on Available-for- Sale Securities
Changes in AOCI by component
Beginning Balance	41,654	35,948	44,057	50,996
OCI before reclassifications	(344)	6,982	(2,747)	(8,066)
Amounts reclassified from AOCI	0	0	0	0
Net period OCI	(344)	6,982	(2,747)	(8,066)
Ending Balance	41,310	42,930	41,310	42,930
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	41,654	35,884	43,993	50,932
OCI before reclassifications	(344)	6,982	(2,747)	(8,066)
Amounts reclassified from AOCI	0	0	64	0
Net period OCI	(344)	6,982	(2,683)	(8,066)
Ending Balance	$ 41,310	$ 42,866	$ 41,310	$ 42,866