Investment Securities - Gross Unrealized Losses (Details) $ in Thousands	Sep. 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	2,074
Securities in unrealized loss positions, qualitative disclosure, number of positions (in security) | security	1,525
Fair Value
Less Than 12 Months	$ 2,032,456	$ 1,379,705
12 Months or More	1,045,212	563,188
Total	3,077,668	1,942,893
Gross Unrealized Losses
Less Than 12 Months	43,491	21,534
12 Months or More	39,693	13,649
Total	83,184	35,183
Government and government-sponsored agencies
Fair Value
Less Than 12 Months	204,095	63,818
12 Months or More	39,573	24,621
Total	243,668	88,439
Gross Unrealized Losses
Less Than 12 Months	2,564	510
12 Months or More	1,640	438
Total	4,204	948
State and political subdivisions
Fair Value
Less Than 12 Months	563,202	437,407
12 Months or More	414,480	349,242
Total	977,682	786,649
Gross Unrealized Losses
Less Than 12 Months	14,773	12,268
12 Months or More	11,819	10,093
Total	26,592	22,361
Residential mortgage-backed securities
Fair Value
Less Than 12 Months	119,911	93,508
12 Months or More	55,068	56,576
Total	174,979	150,084
Gross Unrealized Losses
Less Than 12 Months	2,907	383
12 Months or More	3,061	1,243
Total	5,968	1,626
Collateralized mortgage obligations
Fair Value
Less Than 12 Months	990,167	713,525
12 Months or More	456,347	73,707
Total	1,446,514	787,232
Gross Unrealized Losses
Less Than 12 Months	19,414	7,235
12 Months or More	19,940	1,249
Total	39,354	8,484
Corporate bonds
Fair Value
Less Than 12 Months	141,365	71,447
12 Months or More	76,955	47,878
Total	218,320	119,325
Gross Unrealized Losses
Less Than 12 Months	3,773	1,138
12 Months or More	3,135	454
Total	6,908	1,592
Trust preferred securities
Fair Value
Less Than 12 Months	13,716	0
12 Months or More	2,789	11,164
Total	16,505	11,164
Gross Unrealized Losses
Less Than 12 Months	60	0
12 Months or More	98	172
Total	$ 158	$ 172