Fair Value - Assets and Liabilities Recorded at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Assets
Investment securities	$ 4,842,551	$ 5,234,658
Fair Value Measurements, Recurring
Assets
Investment securities	4,842,551	5,234,658
Other assets	5,384	14,682
Liabilities
Other liabilities	(27,427)	(20,488)
Total
Net Assets (Liabilities)	4,820,508	5,228,852
Fair Value Measurements, Recurring | Level 2
Assets
Investment securities	4,842,551	5,234,658
Other assets	5,384	14,682
Liabilities
Other liabilities	(23,313)	(15,049)
Total
Net Assets (Liabilities)	4,824,622	5,234,291
Fair Value Measurements, Recurring | Level 3
Liabilities
Other liabilities	(4,114)	(5,439)
Total
Net Assets (Liabilities)	(4,114)	(5,439)
U.S. Treasury securities
Assets
Investment securities	386,057	392,255
U.S. Treasury securities | Fair Value Measurements, Recurring
Assets
Investment securities	386,057	392,255
U.S. Treasury securities | Fair Value Measurements, Recurring | Level 2
Assets
Investment securities	386,057	392,255
Government-sponsored enterprises debt securities
Assets
Investment securities	238,527	242,601
Government-sponsored enterprises debt securities | Fair Value Measurements, Recurring
Assets
Investment securities	238,527	242,601
Government-sponsored enterprises debt securities | Fair Value Measurements, Recurring | Level 2
Assets
Investment securities	238,527	242,601
Government agency mortgage-backed securities
Assets
Investment securities	438,850	351,390
Government agency mortgage-backed securities | Fair Value Measurements, Recurring
Assets
Investment securities	438,850	351,390
Government agency mortgage-backed securities | Fair Value Measurements, Recurring | Level 2
Assets
Investment securities	438,850	351,390
Government-sponsored enterprises mortgage-backed securities
Assets
Investment securities	161,241	174,741
Government-sponsored enterprises mortgage-backed securities | Fair Value Measurements, Recurring
Assets
Investment securities	161,241	174,741
Government-sponsored enterprises mortgage-backed securities | Fair Value Measurements, Recurring | Level 2
Assets
Investment securities	161,241	174,741
Collateralized mortgage obligations: Government agency
Assets
Investment securities	2,924,806	3,290,474
Collateralized mortgage obligations: Government agency | Fair Value Measurements, Recurring
Assets
Investment securities	2,924,806	3,290,474
Collateralized mortgage obligations: Government agency | Fair Value Measurements, Recurring | Level 2
Assets
Investment securities	2,924,806	3,290,474
Collateralized mortgage obligations: Government-sponsored enterprises
Assets
Investment securities	673,254	762,718
Collateralized mortgage obligations: Government-sponsored enterprises | Fair Value Measurements, Recurring
Assets
Investment securities	673,254	762,718
Collateralized mortgage obligations: Government-sponsored enterprises | Fair Value Measurements, Recurring | Level 2
Assets
Investment securities	673,254	762,718
States and political subdivisions
Assets
Investment securities	19,816	20,479
States and political subdivisions | Fair Value Measurements, Recurring
Assets
Investment securities	19,816	20,479
States and political subdivisions | Fair Value Measurements, Recurring | Level 2
Assets
Investment securities	$ 19,816	$ 20,479