Investment Securities - Gross Unrealized Losses (Details) $ in Thousands	Jun. 30, 2017 USD ($) security	Dec. 31, 2016 USD ($)
Schedule of Investments [Line Items]
Total number of securities, available-for-sale and held-to-maturity (in security) | security	2,336
Securities in unrealized loss positions, qualitative disclosure, number of positions (in security) | security	1,351
Fair Value
Less Than 12 Months	$ 1,751,829	$ 1,478,240
12 Months or More	70,324	56,897
Total	1,822,153	1,535,137
Gross Unrealized Losses
Less Than 12 Months	26,292	39,116
12 Months or More	1,287	1,751
Total	27,579	40,867
U.S. Treasury securities
Fair Value
Less Than 12 Months	5,793
12 Months or More	0
Total	5,793
Gross Unrealized Losses
Less Than 12 Months	2
12 Months or More	0
Total	2
Government sponsored agencies
Fair Value
Less Than 12 Months	104,655	105,702
12 Months or More	10,816	15,023
Total	115,471	120,725
Gross Unrealized Losses
Less Than 12 Months	1,042	1,707
12 Months or More	124	361
Total	1,166	2,068
State and political subdivisions
Fair Value
Less Than 12 Months	614,276	758,063
12 Months or More	46,588	26,810
Total	660,864	784,873
Gross Unrealized Losses
Less Than 12 Months	15,504	28,158
12 Months or More	838	975
Total	16,342	29,133
Residential mortgage-backed securities
Fair Value
Less Than 12 Months	284,277	244,239
12 Months or More	0	0
Total	284,277	244,239
Gross Unrealized Losses
Less Than 12 Months	4,012	3,992
12 Months or More	0	0
Total	4,012	3,992
Collateralized mortgage obligations
Fair Value
Less Than 12 Months	627,054	279,001
12 Months or More	11,061	14,754
Total	638,115	293,755
Gross Unrealized Losses
Less Than 12 Months	4,765	3,778
12 Months or More	184	225
Total	4,949	4,003
Corporate bonds
Fair Value
Less Than 12 Months	111,931	80,536
12 Months or More	1,499	0
Total	113,430	80,536
Gross Unrealized Losses
Less Than 12 Months	935	1,401
12 Months or More	1	0
Total	936	1,401
Trust preferred securities
Fair Value
Less Than 12 Months	3,843	10,699
12 Months or More	360	310
Total	4,203	11,009
Gross Unrealized Losses
Less Than 12 Months	32	80
12 Months or More	140	190
Total	$ 172	$ 270