Debt Securities Available for Sale - AFS debt securities in a continuous unrealized loss position (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale
Less than 12 months Fair Value	$ 11,462	$ 2,743,002
Less than 12 months Gross Unrealized Losses	119	11,032
12 months or more Fair Value	238,007	2,647,359
12 months or more Gross Unrealized Losses	2,264	28,785
Total Fair Value	249,469	5,390,361
Total Gross Unrealized Losses	2,383	39,817
US Treasury Securities
Debt Securities, Available-for-sale
Less than 12 months Fair Value		2,439,114
Less than 12 months Gross Unrealized Losses		9,798
12 months or more Fair Value		452,784
12 months or more Gross Unrealized Losses		808
Total Fair Value		2,891,898
Total Gross Unrealized Losses		10,606
Obligations of U.S. Government sponsored entities
Debt Securities, Available-for-sale
Less than 12 months Fair Value		9,973
Less than 12 months Gross Unrealized Losses		4
12 months or more Fair Value		99,846
12 months or more Gross Unrealized Losses		107
Total Fair Value		109,819
Total Gross Unrealized Losses		111
Collateralized Mortgage Obligations - Federal agencies
Debt Securities, Available-for-sale
Less than 12 months Fair Value	11,335	114,603
Less than 12 months Gross Unrealized Losses	117	537
12 months or more Fair Value	50,605	310,315
12 months or more Gross Unrealized Losses	1,497	7,016
Total Fair Value	61,940	424,918
Total Gross Unrealized Losses	1,614	7,553
Mortgage Backed Securities
Debt Securities, Available-for-sale
Less than 12 months Fair Value	127	179,312
Less than 12 months Gross Unrealized Losses	2	693
12 months or more Fair Value	187,402	1,784,414
12 months or more Gross Unrealized Losses	767	20,854
Total Fair Value	187,529	1,963,726
Total Gross Unrealized Losses	$ 769	$ 21,547