Investment Securities - Gross Unrealized Losses (Details) $ in Thousands	Jun. 30, 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	2,047
Securities in unrealized loss positions, qualitative disclosure, number of positions (in security) | security	1,474
Fair Value
Less Than 12 Months	$ 2,049,439	$ 1,379,705
12 Months or More	651,927	563,188
Total	2,701,366	1,942,893
Gross Unrealized Losses
Less Than 12 Months	(46,709)	(21,534)
12 Months or More	(20,608)	(13,649)
Total	67,317	35,183
Government and government-sponsored agencies
Fair Value
Less Than 12 Months	173,618	63,818
12 Months or More	31,046	24,621
Total	204,664	88,439
Gross Unrealized Losses
Less Than 12 Months	2,307	510
12 Months or More	1,044	438
Total	3,351	948
State and political subdivisions
Fair Value
Less Than 12 Months	532,178	437,407
12 Months or More	338,016	349,242
Total	870,194	786,649
Gross Unrealized Losses
Less Than 12 Months	(13,619)	(12,268)
12 Months or More	(9,671)	(10,093)
Total	23,290	22,361
Residential mortgage-backed securities
Fair Value
Less Than 12 Months	97,148	93,508
12 Months or More	51,138	56,576
Total	148,286	150,084
Gross Unrealized Losses
Less Than 12 Months	2,203	383
12 Months or More	2,439	1,243
Total	4,642	1,626
Collateralized mortgage obligations
Fair Value
Less Than 12 Months	1,081,106	713,525
12 Months or More	180,809	73,707
Total	1,261,915	787,232
Gross Unrealized Losses
Less Than 12 Months	23,794	7,235
12 Months or More	6,198	1,249
Total	29,992	8,484
Corporate bonds
Fair Value
Less Than 12 Months	163,889	71,447
12 Months or More	48,160	47,878
Total	212,049	119,325
Gross Unrealized Losses
Less Than 12 Months	4,786	1,138
12 Months or More	1,129	454
Total	5,915	1,592
Trust preferred securities
Fair Value
Less Than 12 Months	1,500	0
12 Months or More	2,758	11,164
Total	4,258	11,164
Gross Unrealized Losses
Less Than 12 Months	0	0
12 Months or More	(127)	(172)
Total	$ 127	$ 172