Investment Securities - Gross Unrealized Losses (Details) $ in Thousands	Sep. 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	2,413
Securities in unrealized loss positions, qualitative disclosure, number of positions (in security) | security	1,366
Fair Value
Less Than 12 Months	$ 1,552,088	$ 1,478,240
12 Months or More	359,274	56,897
Total	1,911,362	1,535,137
Gross Unrealized Losses
Less Than 12 Months	(16,648)	(39,116)
12 Months or More	(6,928)	(1,751)
Total	(23,576)	(40,867)
U.S. Treasury securities
Fair Value
Less Than 12 Months	2,999
12 Months or More	0
Total	2,999
Gross Unrealized Losses
Less Than 12 Months	0
12 Months or More	0
Total	0
Government sponsored agencies
Fair Value
Less Than 12 Months	111,672	105,702
12 Months or More	10,265	15,023
Total	121,937	120,725
Gross Unrealized Losses
Less Than 12 Months	(1,266)	(1,707)
12 Months or More	(50)	(361)
Total	(1,316)	(2,068)
State and political subdivisions
Fair Value
Less Than 12 Months	506,514	758,063
12 Months or More	228,910	26,810
Total	735,424	784,873
Gross Unrealized Losses
Less Than 12 Months	(6,659)	(28,158)
12 Months or More	(5,462)	(975)
Total	(12,121)	(29,133)
Residential mortgage-backed securities
Fair Value
Less Than 12 Months	213,620	244,239
12 Months or More	8,677	0
Total	222,297	244,239
Gross Unrealized Losses
Less Than 12 Months	(3,132)	(3,992)
12 Months or More	(155)	0
Total	(3,287)	(3,992)
Collateralized mortgage obligations
Fair Value
Less Than 12 Months	664,682	279,001
12 Months or More	49,104	14,754
Total	713,786	293,755
Gross Unrealized Losses
Less Than 12 Months	(5,089)	(3,778)
12 Months or More	(798)	(225)
Total	(5,887)	(4,003)
Corporate bonds
Fair Value
Less Than 12 Months	45,682	80,536
12 Months or More	58,123	0
Total	103,805	80,536
Gross Unrealized Losses
Less Than 12 Months	(475)	(1,401)
12 Months or More	(282)	0
Total	(757)	(1,401)
Trust preferred securities
Fair Value
Less Than 12 Months	6,919	10,699
12 Months or More	4,195	310
Total	11,114	11,009
Gross Unrealized Losses
Less Than 12 Months	(27)	(80)
12 Months or More	(181)	(190)
Total	$ (208)	$ (270)