Fair Value - Assets and Liabilities Recorded at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Assets
Fair value	$ 5,135,775	$ 4,075,644
Fair Value Measurements, Recurring
Assets
Fair value	5,135,775	4,075,644
Other assets	170,611	63,539
Liabilities
Other liabilities	(3,711)	(4,915)
Total
Net Assets (Liabilities)	5,302,675	4,134,268
Fair Value Measurements, Recurring | Level 2
Assets
Fair value	5,135,775	4,075,644
Other assets	170,611	63,539
Liabilities
Other liabilities	(1,616)	(682)
Total
Net Assets (Liabilities)	5,304,770	4,138,501
Fair Value Measurements, Recurring | Level 3
Liabilities
Other liabilities	(2,095)	(4,233)
Total
Net Assets (Liabilities)	(2,095)	(4,233)
U.S. Treasury and government agency debt securities
Assets
Fair value	94,918	29,888
U.S. Treasury and government agency debt securities | Fair Value Measurements, Recurring
Assets
Fair value	94,918	29,888
U.S. Treasury and government agency debt securities | Fair Value Measurements, Recurring | Level 2
Assets
Fair value	94,918	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	232,617	291,209
Residential - Government agency | Fair Value Measurements, Recurring
Assets
Fair value	232,617	291,209
Residential - Government agency | Fair Value Measurements, Recurring | Level 2
Assets
Fair value	232,617	291,209
Residential - Government-sponsored enterprises
Assets
Fair value	390,912	399,492
Residential - Government-sponsored enterprises | Fair Value Measurements, Recurring
Assets
Fair value	390,912	399,492
Residential - Government-sponsored enterprises | Fair Value Measurements, Recurring | Level 2
Assets
Fair value	390,912	399,492
Commercial - Government agency
Assets
Fair value	629,679
Commercial - Government agency | Fair Value Measurements, Recurring
Assets
Fair value	629,679
Commercial - Government agency | Fair Value Measurements, Recurring | Level 2
Assets
Fair value	629,679
Commercial - Government-sponsored enterprises
Assets
Fair value	322,996	101,719
Commercial - Government-sponsored enterprises | Fair Value Measurements, Recurring
Assets
Fair value	322,996	101,719
Commercial - Government-sponsored enterprises | Fair Value Measurements, Recurring | Level 2
Assets
Fair value	322,996	101,719
Government agency
Assets
Fair value	1,926,584	2,381,278
Government agency | Fair Value Measurements, Recurring
Assets
Fair value	1,926,584	2,381,278
Government agency | Fair Value Measurements, Recurring | Level 2
Assets
Fair value	1,926,584	2,381,278
Government-sponsored enterprises
Assets
Fair value	1,538,069	770,619
Government-sponsored enterprises | Fair Value Measurements, Recurring
Assets
Fair value	1,538,069	770,619
Government-sponsored enterprises | Fair Value Measurements, Recurring | Level 2
Assets
Fair value	$ 1,538,069	$ 770,619