Securities - Additional Information (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Proceeds from sale of debt securities available for sale		$ 3,800,000	$ 0	$ 3,820,000
Gross gains from sale of securities		21,000		21,000
Gross losses from sale of securities		$ 0		$ 0
Decline in value of an investment in shares of mutual funds	$ 100,000		200,000
Unrealized losses from securities transferred from available for sale to held to maturity			3,100,000
Unamortized balance of securities transferred from available for sale to held to maturity	900,000		900,000
Fair value for securities in unrealized loss position for less than twelve months	598,333,000		598,333,000		$ 326,177,000
Unrealized losses on securities in unrealized loss position for less than twelve months	12,937,000		12,937,000		2,134,000
Fair value for securities in unrealized loss position for twelve months or more	507,551,000		507,551,000		440,333,000
Unrealized losses on securities in unrealized loss position for more than twelve months	21,718,000		21,718,000		10,540,000
Unrealized losses on mortgage backed securities of government sponsored entities	34,655,000		34,655,000		12,674,000
Fair value of mortgage backed securities of government sponsored entities	1,105,884,000		1,105,884,000		766,510,000
Other assets	$ 35,000,000		$ 35,000,000		$ 33,000,000
Shares held (in shares)	47,163,109		47,163,109		46,917,735
Ownership basis	0.00%		0.00%
Mutual Fund
Schedule of Available-for-sale Securities [Line Items]
Investment in mutual fund carried at fair value	$ 6,200,000		$ 6,200,000
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	57,700,000		57,700,000		$ 22,900,000
Unrealized losses on securities in unrealized loss position for less than twelve months	1,200,000		1,200,000		200,000
Fair value for securities in unrealized loss position for twelve months or more	13,100,000		13,100,000		15,300,000
Unrealized losses on securities in unrealized loss position for more than twelve months	700,000		700,000		400,000
Carrying Value | Repurchase Agreement
Schedule of Available-for-sale Securities [Line Items]
Fair value of debt securities pledged as collateral	200,100,000		200,100,000
Public and Trust Deposits | Carrying Value
Schedule of Available-for-sale Securities [Line Items]
Fair value of debt securities pledged as collateral	172,700,000		172,700,000
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	420,280,000		420,280,000		304,723,000
Unrealized losses on securities in unrealized loss position for less than twelve months	12,087,000		12,087,000		2,047,000
Fair value for securities in unrealized loss position for twelve months or more	468,753,000		468,753,000		413,725,000
Unrealized losses on securities in unrealized loss position for more than twelve months	20,976,000		20,976,000		10,146,000
Unrealized losses on mortgage backed securities of government sponsored entities	33,063,000		33,063,000		12,193,000
Fair value of mortgage backed securities of government sponsored entities	889,033,000		889,033,000		718,448,000
Collateralized loan obligations
Schedule of Available-for-sale Securities [Line Items]
Unrealized losses on mortgage backed securities of government sponsored entities	500,000		500,000
Fair value of mortgage backed securities of government sponsored entities	138,700,000		138,700,000
Private Collateralized Securities
Schedule of Available-for-sale Securities [Line Items]
Fair value of mortgage backed securities of government sponsored entities	889,000,000		889,000,000
Seasoned Residential Collateral Of Us Government Sponsored Entities
Schedule of Available-for-sale Securities [Line Items]
Unrealized losses on mortgage backed securities of government sponsored entities	500,000		500,000
Fair value of mortgage backed securities of government sponsored entities	44,400,000		44,400,000
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	9,896,000		9,896,000		5,414,000
Unrealized losses on securities in unrealized loss position for less than twelve months	94,000		94,000		14,000
Fair value for securities in unrealized loss position for twelve months or more	20,330,000		20,330,000		5,864,000
Unrealized losses on securities in unrealized loss position for more than twelve months	451,000		451,000		56,000
Unrealized losses on mortgage backed securities of government sponsored entities	545,000		545,000		70,000
Fair value of mortgage backed securities of government sponsored entities	$ 30,226,000		$ 30,226,000		$ 11,278,000
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 (in dollars per share)	$ 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