Securities - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Collateral Underlying Mortgage Investments Terms	30- and 15-year fixed and 10/1 adjustable rate mortgage
Federal Home Loan Bank Stock and Federal Reserve Bank Stock	$ 35,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	978,722		$ 766,510
Accumulated unrealized losses	27,314		12,674
Available For Sale Securities Transferred To Held To Maturity Securities Unrealized Gain Loss	3,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	540,638		326,177
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	10,355		2,134
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	438,084		440,333
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	$ 16,959		$ 10,540
Common Stock, Shares, Outstanding	46,983,165		46,917,735
Marketable Securities, Gain (Loss)	$ 100
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	978,722		$ 766,510
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	27,314		12,674
Mutual Fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Other Assets	6,200
US States and Political Subdivisions Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	31,200
Accumulated unrealized losses	500
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	31,200
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	500
Common Class B [Member]
Schedule of Available-for-sale Securities [Line Items]
Common Stock, Shares, Outstanding		11,330
Common Class A [Member]
Schedule of Available-for-sale Securities [Line Items]
Debt Instrument, Convertible, Conversion Price		$ 1.6483
Stock Issued During Period, Shares, Conversion of Convertible Securities		18,675
Available-for-sale Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	1,100
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	63,300		22,900
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	800		200
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	14,100		15,300
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	600		$ 400
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	1,100
Carrying Amount [Member] | Repurchase Agreement [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities pledged as collateral	173,200
Collateralized Loan Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Accumulated unrealized losses	26,500
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	26,500
Private Collateralized Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	913,500
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	913,500
Seasoned Residential Collateral Of Us Government Sponsored Entities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	20,300
Accumulated unrealized losses	300
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	20,300
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	300
Public and Trust Deposits [Member] | Carrying Amount [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities pledged as collateral	$ 178,800