Debt Securities Available for Sale - AFS debt securities in a continuous unrealized loss position (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale
Less than 12 months Fair Value	$ 10,043,194	$ 890,461
Less than 12 months Gross Unrealized Losses	147,376	1,877
12 months or more Fair Value	1,867	555
12 months or more Gross Unrealized Losses	31	11
Total Fair Value	10,045,061	891,016
Total Gross Unrealized Losses	147,407	1,888
US Treasury Securities
Debt Securities, Available-for-sale
Less than 12 months Fair Value	3,126,340
Less than 12 months Gross Unrealized Losses	15,893
12 months or more Fair Value	0
12 months or more Gross Unrealized Losses	0
Total Fair Value	3,126,340
Total Gross Unrealized Losses	15,893
Collateralized Mortgage Obligations - Federal agencies
Debt Securities, Available-for-sale
Less than 12 months Fair Value	0	4,029
Less than 12 months Gross Unrealized Losses	0	43
12 months or more Fair Value	1,454	0
12 months or more Gross Unrealized Losses	21	0
Total Fair Value	1,454	4,029
Total Gross Unrealized Losses	21	43
Mortgage Backed Securities
Debt Securities, Available-for-sale
Less than 12 months Fair Value	6,916,854	886,432
Less than 12 months Gross Unrealized Losses	131,483	1,834
12 months or more Fair Value	413	555
12 months or more Gross Unrealized Losses	10	11
Total Fair Value	6,917,267	886,987
Total Gross Unrealized Losses	$ 131,493	$ 1,845