Fair Value - Assets and Liabilities Recorded at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Assets
Investment securities	$ 4,595,301	$ 5,234,658
Fair Value Measurements, Recurring
Assets
Investment securities	4,595,301	5,234,658
Other assets	2,044	14,682
Liabilities
Other liabilities	(31,942)	(20,488)
Total
Net Assets (Liabilities)	4,565,403	5,228,852
Fair Value Measurements, Recurring | Level 2
Assets
Investment securities	4,595,301	5,234,658
Other assets	2,044	14,682
Liabilities
Other liabilities	(28,557)	(15,049)
Total
Net Assets (Liabilities)	4,568,788	5,234,291
Fair Value Measurements, Recurring | Level 3
Liabilities
Other liabilities	(3,385)	(5,439)
Total
Net Assets (Liabilities)	(3,385)	(5,439)
U.S. Treasury securities
Assets
Investment securities	383,890	392,255
U.S. Treasury securities | Fair Value Measurements, Recurring
Assets
Investment securities	383,890	392,255
U.S. Treasury securities | Fair Value Measurements, Recurring | Level 2
Assets
Investment securities	383,890	392,255
Government-sponsored enterprises debt securities
Assets
Investment securities	237,217	242,601
Government-sponsored enterprises debt securities | Fair Value Measurements, Recurring
Assets
Investment securities	237,217	242,601
Government-sponsored enterprises debt securities | Fair Value Measurements, Recurring | Level 2
Assets
Investment securities	237,217	242,601
Government agency mortgage-backed securities
Assets
Investment securities	419,293	351,390
Government agency mortgage-backed securities | Fair Value Measurements, Recurring
Assets
Investment securities	419,293	351,390
Government agency mortgage-backed securities | Fair Value Measurements, Recurring | Level 2
Assets
Investment securities	419,293	351,390
Government-sponsored enterprises mortgage-backed securities
Assets
Investment securities	154,133	174,741
Government-sponsored enterprises mortgage-backed securities | Fair Value Measurements, Recurring
Assets
Investment securities	154,133	174,741
Government-sponsored enterprises mortgage-backed securities | Fair Value Measurements, Recurring | Level 2
Assets
Investment securities	154,133	174,741
Collateralized mortgage obligations: Government agency
Assets
Investment securities	2,753,966	3,290,474
Collateralized mortgage obligations: Government agency | Fair Value Measurements, Recurring
Assets
Investment securities	2,753,966	3,290,474
Collateralized mortgage obligations: Government agency | Fair Value Measurements, Recurring | Level 2
Assets
Investment securities	2,753,966	3,290,474
Collateralized mortgage obligations: Government-sponsored enterprises
Assets
Investment securities	627,239	762,718
Collateralized mortgage obligations: Government-sponsored enterprises | Fair Value Measurements, Recurring
Assets
Investment securities	627,239	762,718
Collateralized mortgage obligations: Government-sponsored enterprises | Fair Value Measurements, Recurring | Level 2
Assets
Investment securities	627,239	762,718
States and political subdivisions
Assets
Investment securities	19,563	20,479
States and political subdivisions | Fair Value Measurements, Recurring
Assets
Investment securities	19,563	20,479
States and political subdivisions | Fair Value Measurements, Recurring | Level 2
Assets
Investment securities	$ 19,563	$ 20,479