Derivative Activities - Offsetting of Derivative Assets - Derivative Liabilities, Financial Liabilities, and Collateral Held by Counterparty (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Derivative liabilities:
Net amounts of liabilities presented in the consolidated balance sheet	$ 1,751,532	$ 2,386,662
Financial instruments	(1,738,839)	(2,380,866)
Net amount of liabilities in the consolidated balance sheet	12,693	5,796
Credit Suisse First Boston Mortgage Capital LLC
Derivative liabilities:
Net amounts of liabilities presented in the consolidated balance sheet	628,815	1,010,562
Financial instruments	(628,815)	(1,010,320)
Net amount of liabilities in the consolidated balance sheet		242
Bank of America, N.A.
Derivative liabilities:
Net amounts of liabilities presented in the consolidated balance sheet	253,272	406,787
Financial instruments	(247,572)	(406,355)
Net amount of liabilities in the consolidated balance sheet	5,700	432
Deutsche Bank AG
Derivative liabilities:
Net amounts of liabilities presented in the consolidated balance sheet	598,392	593,864
Financial instruments	(598,392)	(593,864)
Barclays
Derivative liabilities:
Net amounts of liabilities presented in the consolidated balance sheet	388	6,387
Financial instruments		(6,387)
Net amount of liabilities in the consolidated balance sheet	388
Royal Bank of Canada
Derivative liabilities:
Net amounts of liabilities presented in the consolidated balance sheet	33,465	24,835
Financial instruments	(33,465)	(23,752)
Net amount of liabilities in the consolidated balance sheet		1,083
JP Morgan
Derivative liabilities:
Net amounts of liabilities presented in the consolidated balance sheet	59,826	90,442
Financial instruments	(58,780)	(90,442)
Net amount of liabilities in the consolidated balance sheet	1,046
Morgan Stanley Bank
Derivative liabilities:
Net amounts of liabilities presented in the consolidated balance sheet	71,210	139,491
Financial instruments	(71,210)	(138,983)
Net amount of liabilities in the consolidated balance sheet		508
Citibank, N.A.
Derivative liabilities:
Net amounts of liabilities presented in the consolidated balance sheet	19,442	23,010
Financial instruments	(19,442)	(23,010)
BNP Paribas
Derivative liabilities:
Net amounts of liabilities presented in the consolidated balance sheet	81,163	87,753
Financial instruments	(81,163)	(87,753)
Other
Derivative liabilities:
Net amounts of liabilities presented in the consolidated balance sheet	1,647	1,791
Net amount of liabilities in the consolidated balance sheet	1,647	1,791
Interest rate lock commitments
Derivative liabilities:
Net amounts of liabilities presented in the consolidated balance sheet	3,912	1,740
Net amount of liabilities in the consolidated balance sheet	$ 3,912	$ 1,740