Fair Value - Assets and Liabilities Recorded at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Assets
Investment securities	$ 4,498,342	$ 5,234,658
Fair Value Measurements, Recurring
Assets
Investment securities	4,498,342	5,234,658
Other assets	12,336	14,682
Liabilities
Other liabilities	(14,692)	(20,488)
Total
Net Assets (Liabilities)	4,495,986	5,228,852
Fair Value Measurements, Recurring | Level 2
Assets
Investment securities	4,498,342	5,234,658
Other assets	12,336	14,682
Liabilities
Other liabilities	(12,085)	(15,049)
Total
Net Assets (Liabilities)	4,498,593	5,234,291
Fair Value Measurements, Recurring | Level 3
Liabilities
Other liabilities	(2,607)	(5,439)
Total
Net Assets (Liabilities)	(2,607)	(5,439)
U.S. Treasury securities
Assets
Investment securities	389,470	392,255
U.S. Treasury securities | Fair Value Measurements, Recurring
Assets
Investment securities	389,470	392,255
U.S. Treasury securities | Fair Value Measurements, Recurring | Level 2
Assets
Investment securities	389,470	392,255
Government-sponsored enterprises debt securities
Assets
Investment securities	241,594	242,601
Government-sponsored enterprises debt securities | Fair Value Measurements, Recurring
Assets
Investment securities	241,594	242,601
Government-sponsored enterprises debt securities | Fair Value Measurements, Recurring | Level 2
Assets
Investment securities	241,594	242,601
Government agency mortgage-backed securities
Assets
Investment securities	411,536	351,390
Government agency mortgage-backed securities | Fair Value Measurements, Recurring
Assets
Investment securities	411,536	351,390
Government agency mortgage-backed securities | Fair Value Measurements, Recurring | Level 2
Assets
Investment securities	411,536	351,390
Government-sponsored enterprises mortgage-backed securities
Assets
Investment securities	150,847	174,741
Government-sponsored enterprises mortgage-backed securities | Fair Value Measurements, Recurring
Assets
Investment securities	150,847	174,741
Government-sponsored enterprises mortgage-backed securities | Fair Value Measurements, Recurring | Level 2
Assets
Investment securities	150,847	174,741
Collateralized mortgage obligations: Government agency
Assets
Investment securities	2,682,449	3,290,474
Collateralized mortgage obligations: Government agency | Fair Value Measurements, Recurring
Assets
Investment securities	2,682,449	3,290,474
Collateralized mortgage obligations: Government agency | Fair Value Measurements, Recurring | Level 2
Assets
Investment securities	2,682,449	3,290,474
Collateralized mortgage obligations: Government-sponsored enterprises
Assets
Investment securities	602,592	762,718
Collateralized mortgage obligations: Government-sponsored enterprises | Fair Value Measurements, Recurring
Assets
Investment securities	602,592	762,718
Collateralized mortgage obligations: Government-sponsored enterprises | Fair Value Measurements, Recurring | Level 2
Assets
Investment securities	602,592	762,718
States and political subdivisions
Assets
Investment securities	19,854	20,479
States and political subdivisions | Fair Value Measurements, Recurring
Assets
Investment securities	19,854	20,479
States and political subdivisions | Fair Value Measurements, Recurring | Level 2
Assets
Investment securities	$ 19,854	$ 20,479