Investment Securities Narrative (Details) $ in Thousands		6 Months Ended
	Aug. 01, 2018 USD ($)	Jun. 30, 2018 USD ($) security	Dec. 31, 2017 USD ($)	Jun. 30, 2017 USD ($)
Schedule of Investments [Line Items]
Securities pledged as collateral		$ 1,500,000	$ 1,800,000
Amortized cost		2,664,163	2,575,719
Available-for-sale Securities, Fair Value Disclosure		2,593,283	2,547,956
Residential Mortgage Backed Securities [Member]
Schedule of Investments [Line Items]
Amortized cost		1,035,724	1,132,080
Available-for-sale Securities, Fair Value Disclosure		1,000,964	1,120,796
US States and Political Subdivisions Debt Securities [Member]
Schedule of Investments [Line Items]
Amortized cost		415,460	405,860
Available-for-sale Securities, Fair Value Disclosure		407,696	408,949
Equity Securities [Member]
Schedule of Investments [Line Items]
Amortized cost			776
Available-for-sale Securities, Fair Value Disclosure			$ 918
Single-issuer Trust Preferred Securities [Member]
Schedule of Investments [Line Items]
Unrealized Loss on Securities		$ 1,383
Number of Trust Preferred Securities | security		11
External Credit Rating, BBB [Member] | Single-issuer Trust Preferred Securities [Member]
Schedule of Investments [Line Items]
Amortized cost		$ 3,800
Available-for-sale Securities, Fair Value Disclosure		$ 3,200
Not Rated By An External Agency [Member] | Single-issuer Trust Preferred Securities [Member]
Schedule of Investments [Line Items]
Number of Trust Preferred Securities | security		2
External Credit Rating, Rated Below Investment Grade [Member] | Single-issuer Trust Preferred Securities [Member]
Schedule of Investments [Line Items]
Amortized cost		$ 2,000
Available-for-sale Securities, Fair Value Disclosure		$ 1,900
Number of Trust Preferred Securities | security		2
Pooled Trust Preferred Securities [Member]
Schedule of Investments [Line Items]
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Credit Losses on Debt Securities Held		$ 11,500		$ 11,500
Subsequent Event [Member]
Schedule of Investments [Line Items]
Debt securities held-to-maturity, fair value	$ 641,700
Available-for-sale securities, transfers to held-to-maturity, Amortized Cost	665,500
Subsequent Event [Member] | Residential Mortgage Backed Securities [Member]
Schedule of Investments [Line Items]
Debt securities held-to-maturity, fair value	485,300
Available-for-sale securities, transfers to held-to-maturity, Amortized Cost	505,500
Subsequent Event [Member] | US States and Political Subdivisions Debt Securities [Member]
Schedule of Investments [Line Items]
Debt securities held-to-maturity, fair value	156,400
Available-for-sale securities, transfers to held-to-maturity, Amortized Cost	$ 160,000