Investment Securities - Gross Unrealized Losses (Details) $ in Thousands	Jun. 30, 2019 USD ($) security	Dec. 31, 2018 USD ($)
Schedule of Investments [Line Items]
Total number of securities, available-for-sale and held-to-maturity (in security) | security	1,912
Securities in unrealized loss positions, qualitative disclosure, number of positions (in security) | security	512
Fair Value
Less Than 12 Months	$ 369,846	$ 1,078,804
12 Months or More	1,097,973	1,605,905
Total	1,467,819	2,684,709
Gross Unrealized Losses
Less Than 12 Months	2,659	16,485
12 Months or More	12,006	44,904
Total	14,665	61,389
Government and government-sponsored enterprises
Fair Value
Less Than 12 Months	38,077	167,164
12 Months or More	171,701	62,200
Total	209,778	229,364
Gross Unrealized Losses
Less Than 12 Months	74	1,672
12 Months or More	1,862	1,683
Total	1,936	3,355
State and political subdivisions
Fair Value
Less Than 12 Months	10,105	190,551
12 Months or More	242,873	657,327
Total	252,978	847,878
Gross Unrealized Losses
Less Than 12 Months	15	1,932
12 Months or More	1,190	14,016
Total	1,205	15,948
Residential mortgage-backed securities
Fair Value
Less Than 12 Months	14,778	20,679
12 Months or More	102,387	123,757
Total	117,165	144,436
Gross Unrealized Losses
Less Than 12 Months	17	85
12 Months or More	958	3,738
Total	975	3,823
Collateralized mortgage obligations
Fair Value
Less Than 12 Months	229,853	496,356
12 Months or More	464,641	656,208
Total	694,494	1,152,564
Gross Unrealized Losses
Less Than 12 Months	1,684	5,268
12 Months or More	4,189	19,752
Total	5,873	25,020
Corporate bonds
Fair Value
Less Than 12 Months	52,131	169,431
12 Months or More	113,659	103,688
Total	165,790	273,119
Gross Unrealized Losses
Less Than 12 Months	269	5,888
12 Months or More	3,625	5,551
Total	3,894	11,439
Trust preferred securities
Fair Value
Less Than 12 Months	24,902	34,623
12 Months or More	2,712	2,725
Total	27,614	37,348
Gross Unrealized Losses
Less Than 12 Months	600	1,640
12 Months or More	182	164
Total	$ 782	$ 1,804