Investment Securities - Gross Unrealized Losses (Details) $ in Thousands	Mar. 31, 2018 USD ($) security	Dec. 31, 2017 USD ($)
Schedule of Investments [Line Items]
Total number of securities, available-for-sale and held-to-maturity (in security) | security	2,234
Securities in unrealized loss positions, qualitative disclosure, number of positions (in security) | security	1,600
Fair Value
Less Than 12 Months	$ 2,005,310	$ 1,379,705
12 Months or More	557,282	563,188
Total	2,562,592	1,942,893
Gross Unrealized Losses
Less Than 12 Months	(42,748)	(21,534)
12 Months or More	(16,728)	(13,649)
Total	59,476	35,183
Government sponsored agencies
Fair Value
Less Than 12 Months	114,568	63,818
12 Months or More	14,236	24,621
Total	128,804	88,439
Gross Unrealized Losses
Less Than 12 Months	910	510
12 Months or More	764	438
Total	1,674	948
State and political subdivisions
Fair Value
Less Than 12 Months	577,711	437,407
12 Months or More	347,810	349,242
Total	925,521	786,649
Gross Unrealized Losses
Less Than 12 Months	(15,991)	(12,268)
12 Months or More	(10,689)	(10,093)
Total	26,680	22,361
Residential mortgage-backed securities
Fair Value
Less Than 12 Months	89,157	93,508
12 Months or More	53,532	56,576
Total	142,689	150,084
Gross Unrealized Losses
Less Than 12 Months	1,767	383
12 Months or More	2,240	1,243
Total	4,007	1,626
Collateralized mortgage obligations
Fair Value
Less Than 12 Months	1,098,199	713,525
12 Months or More	88,873	73,707
Total	1,187,072	787,232
Gross Unrealized Losses
Less Than 12 Months	20,529	7,235
12 Months or More	1,901	1,249
Total	22,430	8,484
Corporate bonds
Fair Value
Less Than 12 Months	125,675	71,447
12 Months or More	48,557	47,878
Total	174,232	119,325
Gross Unrealized Losses
Less Than 12 Months	3,551	1,138
12 Months or More	1,027	454
Total	4,578	1,592
Trust preferred securities
Fair Value
Less Than 12 Months	0	0
12 Months or More	4,274	11,164
Total	4,274	11,164
Gross Unrealized Losses
Less Than 12 Months	0	0
12 Months or More	(107)	(172)
Total	$ 107	$ 172