Fair Value Measurements on a Recurring Basis (Details) - Recurring Measurements - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Financial Assets
Total assets measured at fair value on a recurring basis	$ 1,886,517	$ 2,425,477
Financial Liabilities
Total liabilities measured at fair value on a recurring basis	12,748	14,725
Interest rate swaps
Financial Liabilities
Interest rate swaps	12,748	14,725
U.S. government and federal agency
Financial Assets
Investment securities, available-for-sale	34,046	39,407
U.S. government sponsored enterprises
Financial Assets
Investment securities, available-for-sale	19,267	19,570
State and local governments
Financial Assets
Investment securities, available-for-sale	641,973	786,373
Corporate bonds
Financial Assets
Investment securities, available-for-sale	249,990	471,951
Residential mortgage-backed securities
Financial Assets
Investment securities, available-for-sale	838,929	1,007,515
Commercial mortgage-backed securities
Financial Assets
Investment securities, available-for-sale	102,312	100,661
Level 1
Financial Assets
Total assets measured at fair value on a recurring basis	0	0
Financial Liabilities
Total liabilities measured at fair value on a recurring basis	0	0
Level 1 | Interest rate swaps
Financial Liabilities
Interest rate swaps	0	0
Level 1 | U.S. government and federal agency
Financial Assets
Investment securities, available-for-sale	0	0
Level 1 | U.S. government sponsored enterprises
Financial Assets
Investment securities, available-for-sale	0	0
Level 1 | State and local governments
Financial Assets
Investment securities, available-for-sale	0	0
Level 1 | Corporate bonds
Financial Assets
Investment securities, available-for-sale	0	0
Level 1 | Residential mortgage-backed securities
Financial Assets
Investment securities, available-for-sale	0	0
Level 1 | Commercial mortgage-backed securities
Financial Assets
Investment securities, available-for-sale	0	0
Level 2
Financial Assets
Total assets measured at fair value on a recurring basis	1,886,517	2,425,477
Financial Liabilities
Total liabilities measured at fair value on a recurring basis	12,748	14,725
Level 2 | Interest rate swaps
Financial Liabilities
Interest rate swaps	12,748	14,725
Level 2 | U.S. government and federal agency
Financial Assets
Investment securities, available-for-sale	34,046	39,407
Level 2 | U.S. government sponsored enterprises
Financial Assets
Investment securities, available-for-sale	19,267	19,570
Level 2 | State and local governments
Financial Assets
Investment securities, available-for-sale	641,973	786,373
Level 2 | Corporate bonds
Financial Assets
Investment securities, available-for-sale	249,990	471,951
Level 2 | Residential mortgage-backed securities
Financial Assets
Investment securities, available-for-sale	838,929	1,007,515
Level 2 | Commercial mortgage-backed securities
Financial Assets
Investment securities, available-for-sale	102,312	100,661
Level 3
Financial Assets
Total assets measured at fair value on a recurring basis	0	0
Financial Liabilities
Total liabilities measured at fair value on a recurring basis	0	0
Level 3 | Interest rate swaps
Financial Liabilities
Interest rate swaps	0	0
Level 3 | U.S. government and federal agency
Financial Assets
Investment securities, available-for-sale	0	0
Level 3 | U.S. government sponsored enterprises
Financial Assets
Investment securities, available-for-sale	0	0
Level 3 | State and local governments
Financial Assets
Investment securities, available-for-sale	0	0
Level 3 | Corporate bonds
Financial Assets
Investment securities, available-for-sale	0	0
Level 3 | Residential mortgage-backed securities
Financial Assets
Investment securities, available-for-sale	0	0
Level 3 | Commercial mortgage-backed securities
Financial Assets
Investment securities, available-for-sale	$ 0	$ 0