Investment Securities - Cost basis, unrealized gains and losses, and fair value of debt securities available for sale (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Debt securities available-for-sale and equity securities:
Amortized Cost	$ 2,660,263	$ 2,623,728
Gross Unrealized Gains	6,454	11,968
Gross Unrealized Losses	38,250	19,846
Fair Value	2,628,467	2,615,850
U.S. Treasuries
Debt securities available-for-sale and equity securities:
Amortized Cost	150,712	122,025
Gross Unrealized Gains	767	0
Gross Unrealized Losses	2,172	912
Fair Value	149,307	121,113
U.S. Government agencies
Debt securities available-for-sale and equity securities:
Amortized Cost	25,493	26,129
Gross Unrealized Gains	335	269
Gross Unrealized Losses	275	26
Fair Value	25,553	26,372
State and political subdivisions
Debt securities available-for-sale and equity securities:
Amortized Cost	234,750	195,663
Gross Unrealized Gains	907	2,019
Gross Unrealized Losses	1,716	396
Fair Value	233,941	197,286
Residential mortgage-backed securities
Debt securities available-for-sale and equity securities:
Amortized Cost	1,464,380	1,317,371
Gross Unrealized Gains	3,428	7,058
Gross Unrealized Losses	21,898	12,696
Fair Value	1,445,910	1,311,733
Commercial mortgage-backed securities
Debt securities available-for-sale and equity securities:
Amortized Cost	399,663	420,685
Gross Unrealized Gains	187	31
Gross Unrealized Losses	7,933	5,238
Fair Value	391,917	415,478
Corporate bonds
Debt securities available-for-sale and equity securities:
Amortized Cost	200,582	305,265
Gross Unrealized Gains	502	1,513
Gross Unrealized Losses	1,921	425
Fair Value	199,163	306,353
Asset-backed securities
Debt securities available-for-sale and equity securities:
Amortized Cost	184,683	236,533
Gross Unrealized Gains	328	1,078
Gross Unrealized Losses	2,335	153
Fair Value	$ 182,676	237,458
Other
Debt securities available-for-sale and equity securities:
Amortized Cost		57
Gross Unrealized Gains		0
Gross Unrealized Losses		0
Fair Value		$ 57

Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.

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This item represents the cost of debt securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
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Amount of debt securities categorized neither as held-to-maturity nor trading.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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