Derivative Financial Instruments - Offsetting of Derivative Assets - Derivative Liabilities, Financial Liabilities, and Collateral Held by Counterparty (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Derivative liabilities:
Net amounts of liabilities presented in the consolidated balance sheet	$ 3,553,494	$ 1,938,264
Financial instruments	(3,539,459)	(1,935,200)
Net amount of liabilities in the consolidated balance sheet	14,035	3,064
Credit Suisse First Boston Mortgage Capital LLC
Derivative liabilities:
Net amounts of liabilities presented in the consolidated balance sheet	1,000,098	691,030
Financial instruments	(999,713)	(690,766)
Net amount of liabilities in the consolidated balance sheet	385	264
Bank of America, N.A.
Derivative liabilities:
Net amounts of liabilities presented in the consolidated balance sheet	506,553	170,820
Financial instruments	(497,335)	(170,820)
Net amount of liabilities in the consolidated balance sheet	9,218
Deutsche Bank AG
Derivative liabilities:
Net amounts of liabilities presented in the consolidated balance sheet		741,978
Financial instruments		(741,978)
Royal Bank of Canada
Derivative liabilities:
Net amounts of liabilities presented in the consolidated balance sheet	125,735	35,181
Financial instruments	(125,735)	(35,181)
JP Morgan
Derivative liabilities:
Net amounts of liabilities presented in the consolidated balance sheet	711,619	54,326
Financial instruments	(711,619)	(54,326)
Morgan Stanley Bank
Derivative liabilities:
Net amounts of liabilities presented in the consolidated balance sheet	551,027	77,687
Financial instruments	(551,027)	(77,687)
Citibank, N.A.
Derivative liabilities:
Net amounts of liabilities presented in the consolidated balance sheet	461,496	14,960
Financial instruments	(461,496)	(14,960)
Federal National Mortgage Association
Derivative liabilities:
Net amounts of liabilities presented in the consolidated balance sheet	685
Net amount of liabilities in the consolidated balance sheet	685
BNP Paribas
Derivative liabilities:
Net amounts of liabilities presented in the consolidated balance sheet	192,534	149,675
Financial instruments	(192,534)	(149,482)
Net amount of liabilities in the consolidated balance sheet		193
Other
Derivative liabilities:
Net amounts of liabilities presented in the consolidated balance sheet	1,471	1,438
Net amount of liabilities in the consolidated balance sheet	1,471	1,438
Interest rate lock commitments
Derivative liabilities:
Net amounts of liabilities presented in the consolidated balance sheet	2,276	1,169
Net amount of liabilities in the consolidated balance sheet	$ 2,276	$ 1,169