Fair Value - Assets and Liabilities Recorded at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Assets
Investment securities	$ 5,234,658	$ 5,077,514
Fair Value Measurements, Recurring
Assets
Investment securities	5,234,658	5,077,514
Other assets	14,682	15,982
Liabilities
Other liabilities	(20,488)	(31,202)
Total
Net Assets (Liabilities)	5,228,852	5,062,294
Fair Value Measurements, Recurring | Level 2
Assets
Investment securities	5,234,658	5,077,514
Other assets	14,682	15,982
Liabilities
Other liabilities	(15,049)	(23,742)
Total
Net Assets (Liabilities)	5,234,291	5,069,754
Fair Value Measurements, Recurring | Level 3
Liabilities
Other liabilities	(5,439)	(7,460)
Total
Net Assets (Liabilities)	(5,439)	(7,460)
U.S. Treasury securities
Assets
Investment securities	392,255	392,473
U.S. Treasury securities | Fair Value Measurements, Recurring
Assets
Investment securities	392,255	392,473
U.S. Treasury securities | Fair Value Measurements, Recurring | Level 2
Assets
Investment securities	392,255	392,473
Government-sponsored enterprises debt securities
Assets
Investment securities	242,601	242,667
Government-sponsored enterprises debt securities | Fair Value Measurements, Recurring
Assets
Investment securities	242,601	242,667
Government-sponsored enterprises debt securities | Fair Value Measurements, Recurring | Level 2
Assets
Investment securities	242,601	242,667
Government agency mortgage-backed securities
Assets
Investment securities	351,390	185,663
Government agency mortgage-backed securities | Fair Value Measurements, Recurring
Assets
Investment securities	351,390	185,663
Government agency mortgage-backed securities | Fair Value Measurements, Recurring | Level 2
Assets
Investment securities	351,390	185,663
Government-sponsored enterprises mortgage-backed securities
Assets
Investment securities	174,741	204,385
Government-sponsored enterprises mortgage-backed securities | Fair Value Measurements, Recurring
Assets
Investment securities	174,741	204,385
Government-sponsored enterprises mortgage-backed securities | Fair Value Measurements, Recurring | Level 2
Assets
Investment securities	174,741	204,385
Non-government asset-backed securities
Assets
Investment securities		12,583
Non-government asset-backed securities | Fair Value Measurements, Recurring
Assets
Investment securities		12,583
Non-government asset-backed securities | Fair Value Measurements, Recurring | Level 2
Assets
Investment securities		12,583
Collateralized mortgage obligations: Government agency
Assets
Investment securities	3,290,474	3,351,822
Collateralized mortgage obligations: Government agency | Fair Value Measurements, Recurring
Assets
Investment securities	3,290,474	3,351,822
Collateralized mortgage obligations: Government agency | Fair Value Measurements, Recurring | Level 2
Assets
Investment securities	3,290,474	3,351,822
Collateralized mortgage obligations: Government-sponsored enterprises
Assets
Investment securities	762,718	687,921
Collateralized mortgage obligations: Government-sponsored enterprises | Fair Value Measurements, Recurring
Assets
Investment securities	762,718	687,921
Collateralized mortgage obligations: Government-sponsored enterprises | Fair Value Measurements, Recurring | Level 2
Assets
Investment securities	762,718	$ 687,921
States and political subdivisions
Assets
Investment securities	20,479
States and political subdivisions | Fair Value Measurements, Recurring
Assets
Investment securities	20,479
States and political subdivisions | Fair Value Measurements, Recurring | Level 2
Assets
Investment securities	$ 20,479