Debt Securities Available for Sale - AFS debt securities in a continuous unrealized loss position (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale
Less than 12 months Fair Value	$ 11,373,710	$ 890,461
Less than 12 months Gross Unrealized Losses	163,663	1,877
12 months or more Fair Value	1,466	555
12 months or more Gross Unrealized Losses	25	11
Total Fair Value	11,375,176	891,016
Total Gross Unrealized Losses	163,688	1,888
US Treasury Securities
Debt Securities, Available-for-sale
Less than 12 months Fair Value	4,607,689
Less than 12 months Gross Unrealized Losses	22,936
12 months or more Fair Value	0
12 months or more Gross Unrealized Losses	0
Total Fair Value	4,607,689
Total Gross Unrealized Losses	22,936
Collateralized Mortgage Obligations - Federal agencies
Debt Securities, Available-for-sale
Less than 12 months Fair Value	22,197	4,029
Less than 12 months Gross Unrealized Losses	56	43
12 months or more Fair Value	1,124	0
12 months or more Gross Unrealized Losses	15	0
Total Fair Value	23,321	4,029
Total Gross Unrealized Losses	71	43
Mortgage Backed Securities
Debt Securities, Available-for-sale
Less than 12 months Fair Value	6,743,824	886,432
Less than 12 months Gross Unrealized Losses	140,671	1,834
12 months or more Fair Value	342	555
12 months or more Gross Unrealized Losses	10	11
Total Fair Value	6,744,166	886,987
Total Gross Unrealized Losses	$ 140,681	$ 1,845