Fair Value - Assets and Liabilities Recorded at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Assets
Fair value	$ 4,058,457	$ 4,075,644
Fair Value Measurements, Recurring
Assets
Fair value	4,058,457	4,075,644
Other assets	158,711	63,539
Liabilities
Other liabilities	(4,886)	(4,915)
Total
Net Assets (Liabilities)	4,212,282	4,134,268
Fair Value Measurements, Recurring | Level 2
Assets
Fair value	4,058,457	4,075,644
Other assets	158,711	63,539
Liabilities
Other liabilities	(1,687)	(682)
Total
Net Assets (Liabilities)	4,215,481	4,138,501
Fair Value Measurements, Recurring | Level 3
Liabilities
Other liabilities	(3,199)	(4,233)
Total
Net Assets (Liabilities)	(3,199)	(4,233)
U.S. Treasury securities
Assets
Fair value	30,927	29,888
U.S. Treasury securities | Fair Value Measurements, Recurring
Assets
Fair value	30,927	29,888
U.S. Treasury securities | Fair Value Measurements, Recurring | Level 2
Assets
Fair value	30,927	29,888
Government-sponsored enterprises debt securities
Assets
Fair value	26,721	101,439
Government-sponsored enterprises debt securities | Fair Value Measurements, Recurring
Assets
Fair value	26,721	101,439
Government-sponsored enterprises debt securities | Fair Value Measurements, Recurring | Level 2
Assets
Fair value	26,721	101,439
Residential - Government agency
Assets
Fair value	278,963	291,209
Residential - Government agency | Fair Value Measurements, Recurring
Assets
Fair value	278,963	291,209
Residential - Government agency | Fair Value Measurements, Recurring | Level 2
Assets
Fair value	278,963	291,209
Residential - Government-sponsored enterprises
Assets
Fair value	417,667	399,492
Residential - Government-sponsored enterprises | Fair Value Measurements, Recurring
Assets
Fair value	417,667	399,492
Residential - Government-sponsored enterprises | Fair Value Measurements, Recurring | Level 2
Assets
Fair value	417,667	399,492
Commercial - Government-sponsored enterprises
Assets
Fair value	186,784	101,719
Commercial - Government-sponsored enterprises | Fair Value Measurements, Recurring
Assets
Fair value	186,784	101,719
Commercial - Government-sponsored enterprises | Fair Value Measurements, Recurring | Level 2
Assets
Fair value	186,784	101,719
Government agency
Assets
Fair value	2,320,878	2,381,278
Government agency | Fair Value Measurements, Recurring
Assets
Fair value	2,320,878	2,381,278
Government agency | Fair Value Measurements, Recurring | Level 2
Assets
Fair value	2,320,878	2,381,278
Government-sponsored enterprises
Assets
Fair value	796,517	770,619
Government-sponsored enterprises | Fair Value Measurements, Recurring
Assets
Fair value	796,517	770,619
Government-sponsored enterprises | Fair Value Measurements, Recurring | Level 2
Assets
Fair value	$ 796,517	$ 770,619