Fair Value - Assets and Liabilities Recorded at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Assets
Investment securities	$ 4,075,644	$ 4,498,342
Fair Value Measurements, Recurring
Assets
Investment securities	4,075,644	4,498,342
Other assets	63,539	12,336
Liabilities
Other liabilities	(4,915)	(14,692)
Total
Net Assets (Liabilities)	4,134,268	4,495,986
Fair Value Measurements, Recurring | Level 2
Assets
Investment securities	4,075,644	4,498,342
Other assets	63,539	12,336
Liabilities
Other liabilities	(682)	(12,085)
Total
Net Assets (Liabilities)	4,138,501	4,498,593
Fair Value Measurements, Recurring | Level 3
Liabilities
Other liabilities	(4,233)	(2,607)
Total
Net Assets (Liabilities)	(4,233)	(2,607)
U.S. Treasury securities
Assets
Investment securities	29,888	389,470
U.S. Treasury securities | Fair Value Measurements, Recurring
Assets
Investment securities	29,888	389,470
U.S. Treasury securities | Fair Value Measurements, Recurring | Level 2
Assets
Investment securities	29,888	389,470
Government-sponsored enterprises debt securities
Assets
Investment securities	101,439	241,594
Government-sponsored enterprises debt securities | Fair Value Measurements, Recurring
Assets
Investment securities	101,439	241,594
Government-sponsored enterprises debt securities | Fair Value Measurements, Recurring | Level 2
Assets
Investment securities	101,439	241,594
Residential - Government agency
Assets
Investment securities	291,209	411,536
Residential - Government agency | Fair Value Measurements, Recurring
Assets
Investment securities	291,209	411,536
Residential - Government agency | Fair Value Measurements, Recurring | Level 2
Assets
Investment securities	291,209	411,536
Residential - Government-sponsored enterprises
Assets
Investment securities	399,492	150,847
Residential - Government-sponsored enterprises | Fair Value Measurements, Recurring
Assets
Investment securities	399,492	150,847
Residential - Government-sponsored enterprises | Fair Value Measurements, Recurring | Level 2
Assets
Investment securities	399,492	150,847
Commercial - Government-sponsored enterprises
Assets
Investment securities	101,719
Commercial - Government-sponsored enterprises | Fair Value Measurements, Recurring
Assets
Investment securities	101,719
Commercial - Government-sponsored enterprises | Fair Value Measurements, Recurring | Level 2
Assets
Investment securities	101,719
Government agency
Assets
Investment securities	2,381,278	2,682,449
Government agency | Fair Value Measurements, Recurring
Assets
Investment securities	2,381,278	2,682,449
Government agency | Fair Value Measurements, Recurring | Level 2
Assets
Investment securities	2,381,278	2,682,449
Government-sponsored enterprises
Assets
Investment securities	770,619	602,592
Government-sponsored enterprises | Fair Value Measurements, Recurring
Assets
Investment securities	770,619	602,592
Government-sponsored enterprises | Fair Value Measurements, Recurring | Level 2
Assets
Investment securities	$ 770,619	602,592
Debt securities issued by states and political subdivisions
Assets
Investment securities		19,854
Debt securities issued by states and political subdivisions | Fair Value Measurements, Recurring
Assets
Investment securities		19,854
Debt securities issued by states and political subdivisions | Fair Value Measurements, Recurring | Level 2
Assets
Investment securities		$ 19,854