Securities - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	12 Months Ended
	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Proceeds from Sale of Available-for-sale Securities, Debt		$ 235,613	$ 40,421	$ 60,314
Available-for-sale Securities, Gross Realized Gains		1,600	500	700
Available-for-sale Securities, Gross Realized Losses		$ 1,500	100	500
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		$ 32,500
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value		773,010	862,873
Accumulated unrealized losses		12,831	18,443
Securities Available For Sale Transferred To Held-To-Maturity					$ 158,800
Available For Sale Securities Transferred To Held To Maturity Securities Unrealized Gain Loss					$ 3,100
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value		332,677	664,476
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		2,290	13,451
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value		440,333	198,397
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		$ 10,541	$ 4,992
Common Stock, Shares, Outstanding		46,917,735	38,021,835
Debt Instrument, Convertible, Conversion Price		$ 1.6483
Number of shares acquired	200,000
Payments to Acquire Marketable Securities		$ 6,180	$ 0	0
Gain (Loss) on Sale of Securities, Net		$ 15,153	0	$ 0
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]
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		$ 1,200
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value		22,900	22,800
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		200	$ 400
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		400
Carrying Amount [Member] | Repurchase Agreement [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities pledged as collateral		216,100
Carrying Amount [Member] | United States Treasury Deposits and Other Public and Trust Deposits [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities pledged as collateral		169,000
Collateralized Loan Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value		718,400
Accumulated unrealized losses		12,200
Private Collateralized Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value		20,800
Accumulated unrealized losses		$ 300