Derivative Financial Instruments - Additional Information (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) counterparty	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Derivative [Line Items]
Notional Amount	$ 6,644,081	$ 4,980,343
Net unrealized gain (loss) on interest rate agreements	1,022	3,271	$ (1,409)
Stockholders' equity attributable to parent	$ 1,212,287	1,149,428	1,146,265	$ 1,265,869
Number of counterparties | counterparty	3
Net Unrealized Losses on Interest Rate Agreements Accounted for as Cash Flow Hedges
Derivative [Line Items]
Stockholders' equity attributable to parent	$ (42,953)	(44,020)	(47,363)
Derivative Liabilities
Derivative [Line Items]
Notional Amount	3,485,466	2,423,362
Mortgage Banking Activities, Net
Derivative [Line Items]
Changes in fair value of assets	51,844	36,445	6,482
Loan purchase commitments | Mortgage Banking Activities, Net
Derivative [Line Items]
Changes in fair value of assets	37,880	25,613	50,234
Interest Rate Contract
Derivative [Line Items]
Notional Amount	3,210,000	2,460,000
TBAs
Derivative [Line Items]
Notional Amount	1,700,000	1,360,000
TBAs | Derivative Liabilities
Derivative [Line Items]
Notional Amount	1,400,000	510,000
Futures
Derivative [Line Items]
Notional Amount	8,000	88,000
Futures | Derivative Liabilities
Derivative [Line Items]
Notional Amount	0	87,500
Residential and Commercial Loans
Derivative [Line Items]
Valuation adjustments on derivatives	(31,000)	10,000	$ (65,000)
Interest rate agreements | Cash Flow Hedging
Derivative [Line Items]
Net unrealized gain (loss) on interest rate agreements	1,000	3,000
Interest rate agreements | Derivative Liabilities
Derivative [Line Items]
Notional Amount	1,248,000	1,101,500
Interest rate agreements | Derivative Liabilities | Cash Flow Hedging
Derivative [Line Items]
Notional Amount	$ 139,500	$ 139,500