Debt Securities Available for Sale - AFS debt securities in a continuous unrealized loss position (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale
Less than 12 months Fair Value	$ 12,236,691	$ 890,461
Less than 12 months Gross Unrealized Losses	251,090	1,877
12 months or more Fair Value	2,416	555
12 months or more Gross Unrealized Losses	41	11
Total Fair Value	12,239,107	891,016
Total Gross Unrealized Losses	251,131	1,888
US Treasury Securities
Debt Securities, Available-for-sale
Less than 12 months Fair Value	4,974,081
Less than 12 months Gross Unrealized Losses	49,173
12 months or more Fair Value	0
12 months or more Gross Unrealized Losses	0
Total Fair Value	4,974,081
Total Gross Unrealized Losses	49,173
Collateralized Mortgage Obligations - Federal agencies
Debt Securities, Available-for-sale
Less than 12 months Fair Value	37,475	4,029
Less than 12 months Gross Unrealized Losses	75	43
12 months or more Fair Value	1,808	0
12 months or more Gross Unrealized Losses	29	0
Total Fair Value	39,283	4,029
Total Gross Unrealized Losses	104	43
Mortgage Backed Securities
Debt Securities, Available-for-sale
Less than 12 months Fair Value	7,225,135	886,432
Less than 12 months Gross Unrealized Losses	201,842	1,834
12 months or more Fair Value	608	555
12 months or more Gross Unrealized Losses	12	11
Total Fair Value	7,225,743	886,987
Total Gross Unrealized Losses	$ 201,854	$ 1,845