Securities - Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Proceeds from sale of debt securities available for sale		$ 3,700	$ 0	$ 7,525
Gross gains from sale of securities		15		36
Gross losses from sale of securities		$ 62		$ 62
Gain (loss) in investment in shares of mutual funds	$ (100)		(200)
Unrealized losses from securities transferred from available for sale to held to maturity			3,100
Unamortized balance of securities transferred from available for sale to held to maturity	800		800
Fair value for securities in unrealized loss position for less than twelve months	843,091		843,091		$ 326,177
Unrealized losses on securities in unrealized loss position for less than twelve months	17,268		17,268		2,134
Fair value for securities in unrealized loss position for twelve months or more	434,034		434,034		440,333
Unrealized losses on securities in unrealized loss position for more than twelve months	22,853		22,853		10,540
Unrealized losses on mortgage backed securities of government sponsored entities	(40,121)		(40,121)		(12,674)
Fair value of mortgage backed securities of government sponsored entities	1,277,125		1,277,125		766,510
Other assets	37,500		37,500		$ 32,500
Investment in mutual fund carried at fair value	$ 6,100		$ 6,100
Shares held (in shares)	47,269,692		47,269,692		46,917,735
Ownership basis	0.00%		0.00%
Mortgage-backed securities and collateralized mortgage obligations of U.S. Government Sponsored Entities
Schedule of Available-for-sale Securities [Line Items]
Fair value for securities in unrealized loss position for less than twelve months	$ 456,356		$ 456,356		$ 304,723
Unrealized losses on securities in unrealized loss position for less than twelve months	15,703		15,703		2,047
Fair value for securities in unrealized loss position for twelve months or more	416,936		416,936		413,725
Unrealized losses on securities in unrealized loss position for more than twelve months	22,514		22,514		10,146
Unrealized losses on mortgage backed securities of government sponsored entities	(38,217)		(38,217)		(12,193)
Fair value of mortgage backed securities of government sponsored entities	873,292		873,292		718,448
Private collateralized mortgage obligations
Schedule of Available-for-sale Securities [Line Items]
Fair value for securities in unrealized loss position for less than twelve months	84,026		84,026		0
Unrealized losses on securities in unrealized loss position for less than twelve months	251		251		0
Fair value for securities in unrealized loss position for twelve months or more	13,836		13,836		20,744
Unrealized losses on securities in unrealized loss position for more than twelve months	197		197		338
Unrealized losses on mortgage backed securities of government sponsored entities	(448)		(448)		(338)
Fair value of mortgage backed securities of government sponsored entities	97,862		97,862		20,744
Collateralized loan obligations
Schedule of Available-for-sale Securities [Line Items]
Unrealized losses on mortgage backed securities of government sponsored entities	(600)		(600)
Fair value of mortgage backed securities of government sponsored entities	255,000		255,000
Obligations of state and political subdivisions
Schedule of Available-for-sale Securities [Line Items]
Fair value for securities in unrealized loss position for less than twelve months	40,136		40,136		5,414
Unrealized losses on securities in unrealized loss position for less than twelve months	672		672		14
Fair value for securities in unrealized loss position for twelve months or more	3,262		3,262		5,864
Unrealized losses on securities in unrealized loss position for more than twelve months	142		142		56
Unrealized losses on mortgage backed securities of government sponsored entities	(814)		(814)		(70)
Fair value of mortgage backed securities of government sponsored entities	43,398		43,398		11,278
Available-for-sale Securities
Schedule of Available-for-sale Securities [Line Items]
Fair value for securities in unrealized loss position for less than twelve months	53,600		53,600		22,900
Unrealized losses on securities in unrealized loss position for less than twelve months	1,500		1,500		200
Fair value for securities in unrealized loss position for twelve months or more	14,700		14,700		15,300
Unrealized losses on securities in unrealized loss position for more than twelve months	900		900		400
Carrying Value | Public and Trust Deposits
Schedule of Available-for-sale Securities [Line Items]
Fair value of debt securities pledged as collateral	162,400		162,400
Carrying Value | Repurchase Agreement
Schedule of Available-for-sale Securities [Line Items]
Fair value of debt securities pledged as collateral	$ 189,000		$ 189,000
Visa | Common Class B
Schedule of Available-for-sale Securities [Line Items]
Shares held (in shares)	11,330		11,330
Visa | Common Class A
Schedule of Available-for-sale Securities [Line Items]
Conversion rate of Class A stock for each share of Class B stock (shares)	$ 1.6298		$ 1.6298
Shares issued (in shares)			18,465
Maximum | Seasoned Residential Collateral Of Us Government Sponsored Entities
Schedule of Available-for-sale Securities [Line Items]
Mortgage term			30 years
Minimum | Seasoned Residential Collateral Of Us Government Sponsored Entities
Schedule of Available-for-sale Securities [Line Items]
Mortgage term			15 years
Collateralized loan obligations
Schedule of Available-for-sale Securities [Line Items]
Fair value for securities in unrealized loss position for less than twelve months	$ 255,030		$ 255,030		14,933
Unrealized losses on securities in unrealized loss position for less than twelve months	585		585		68
Fair value for securities in unrealized loss position for twelve months or more	0		0		0
Unrealized losses on securities in unrealized loss position for more than twelve months	0		0		0
Unrealized losses on mortgage backed securities of government sponsored entities	(585)		(585)		(68)
Fair value of mortgage backed securities of government sponsored entities	$ 255,030		$ 255,030		$ 14,933