Fair Value - Assets and Liabilities Recorded at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Assets
Fair value	$ 5,692,883	$ 4,075,644
Fair Value Measurements, Recurring
Assets
Fair value	5,692,883	4,075,644
Other assets	159,884	63,539
Liabilities
Other liabilities	(2,354)	(4,915)
Total
Net Assets (Liabilities)	5,850,413	4,134,268
Fair Value Measurements, Recurring | Level 2
Assets
Fair value	5,692,883	4,075,644
Other assets	159,884	63,539
Liabilities
Other liabilities	(1,434)	(682)
Total
Net Assets (Liabilities)	5,851,333	4,138,501
Fair Value Measurements, Recurring | Level 3
Liabilities
Other liabilities	(920)	(4,233)
Total
Net Assets (Liabilities)	(920)	(4,233)
U.S. Treasury and government agency debt securities
Assets
Fair value	144,956	29,888
U.S. Treasury and government agency debt securities | Fair Value Measurements, Recurring
Assets
Fair value	144,956
U.S. Treasury and government agency debt securities | Fair Value Measurements, Recurring | Level 2
Assets
Fair value	144,956
U.S. Treasury securities | Fair Value Measurements, Recurring
Assets
Fair value		29,888
U.S. Treasury securities | Fair Value Measurements, Recurring | Level 2
Assets
Fair value		29,888
Government-sponsored enterprises debt securities
Assets
Fair value		101,439
Government-sponsored enterprises debt securities | Fair Value Measurements, Recurring
Assets
Fair value		101,439
Government-sponsored enterprises debt securities | Fair Value Measurements, Recurring | Level 2
Assets
Fair value		101,439
Residential - Government agency
Assets
Fair value	193,948	291,209
Residential - Government agency | Fair Value Measurements, Recurring
Assets
Fair value	193,948	291,209
Residential - Government agency | Fair Value Measurements, Recurring | Level 2
Assets
Fair value	193,948	291,209
Residential - Government-sponsored enterprises
Assets
Fair value	431,842	399,492
Residential - Government-sponsored enterprises | Fair Value Measurements, Recurring
Assets
Fair value	431,842	399,492
Residential - Government-sponsored enterprises | Fair Value Measurements, Recurring | Level 2
Assets
Fair value	431,842	399,492
Commercial - Government agency
Assets
Fair value	751,922
Commercial - Government agency | Fair Value Measurements, Recurring
Assets
Fair value	751,922
Commercial - Government agency | Fair Value Measurements, Recurring | Level 2
Assets
Fair value	751,922
Commercial - Government-sponsored enterprises
Assets
Fair value	498,273	101,719
Commercial - Government-sponsored enterprises | Fair Value Measurements, Recurring
Assets
Fair value	498,273	101,719
Commercial - Government-sponsored enterprises | Fair Value Measurements, Recurring | Level 2
Assets
Fair value	498,273	101,719
Government agency
Assets
Fair value	2,085,976	2,381,278
Government agency | Fair Value Measurements, Recurring
Assets
Fair value	2,085,976	2,381,278
Government agency | Fair Value Measurements, Recurring | Level 2
Assets
Fair value	2,085,976	2,381,278
Government-sponsored enterprises
Assets
Fair value	1,585,966	770,619
Government-sponsored enterprises | Fair Value Measurements, Recurring
Assets
Fair value	1,585,966	770,619
Government-sponsored enterprises | Fair Value Measurements, Recurring | Level 2
Assets
Fair value	$ 1,585,966	$ 770,619