Fair Value - Assets and Liabilities Recorded at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Assets
Investment securities	$ 5,076,766	$ 5,234,658
Fair Value Measurements, Recurring
Assets
Investment securities	5,076,766	5,234,658
Other assets	3,967	14,682
Liabilities
Other liabilities	(22,495)	(20,488)
Total
Net Assets (Liabilities)	5,058,238	5,228,852
Fair Value Measurements, Recurring | Level 2
Assets
Investment securities	5,076,766	5,234,658
Other assets	3,967	14,682
Liabilities
Other liabilities	(17,650)	(15,049)
Total
Net Assets (Liabilities)	5,063,083	5,234,291
Fair Value Measurements, Recurring | Level 3
Liabilities
Other liabilities	(4,845)	(5,439)
Total
Net Assets (Liabilities)	(4,845)	(5,439)
U.S. Treasury securities
Assets
Investment securities	387,647	392,255
U.S. Treasury securities | Fair Value Measurements, Recurring
Assets
Investment securities	387,647	392,255
U.S. Treasury securities | Fair Value Measurements, Recurring | Level 2
Assets
Investment securities	387,647	392,255
Government-sponsored enterprises debt securities
Assets
Investment securities	239,655	242,601
Government-sponsored enterprises debt securities | Fair Value Measurements, Recurring
Assets
Investment securities	239,655	242,601
Government-sponsored enterprises debt securities | Fair Value Measurements, Recurring | Level 2
Assets
Investment securities	239,655	242,601
Government agency mortgage-backed securities
Assets
Investment securities	456,308	351,390
Government agency mortgage-backed securities | Fair Value Measurements, Recurring
Assets
Investment securities	456,308	351,390
Government agency mortgage-backed securities | Fair Value Measurements, Recurring | Level 2
Assets
Investment securities	456,308	351,390
Government-sponsored enterprises mortgage-backed securities
Assets
Investment securities	168,049	174,741
Government-sponsored enterprises mortgage-backed securities | Fair Value Measurements, Recurring
Assets
Investment securities	168,049	174,741
Government-sponsored enterprises mortgage-backed securities | Fair Value Measurements, Recurring | Level 2
Assets
Investment securities	168,049	174,741
Collateralized mortgage obligations: Government agency
Assets
Investment securities	3,087,708	3,290,474
Collateralized mortgage obligations: Government agency | Fair Value Measurements, Recurring
Assets
Investment securities	3,087,708	3,290,474
Collateralized mortgage obligations: Government agency | Fair Value Measurements, Recurring | Level 2
Assets
Investment securities	3,087,708	3,290,474
Collateralized mortgage obligations: Government-sponsored enterprises
Assets
Investment securities	717,483	762,718
Collateralized mortgage obligations: Government-sponsored enterprises | Fair Value Measurements, Recurring
Assets
Investment securities	717,483	762,718
Collateralized mortgage obligations: Government-sponsored enterprises | Fair Value Measurements, Recurring | Level 2
Assets
Investment securities	717,483	762,718
States and political subdivisions
Assets
Investment securities	19,916	20,479
States and political subdivisions | Fair Value Measurements, Recurring
Assets
Investment securities	19,916	20,479
States and political subdivisions | Fair Value Measurements, Recurring | Level 2
Assets
Investment securities	$ 19,916	$ 20,479