Derivative Activities - Offsetting of Derivative Assets - Derivative Liabilities, Financial Liabilities, and Collateral Held by Counterparty (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Derivative liabilities:
Net amounts of liabilities presented in the consolidated balance sheet	$ 2,170,426	$ 1,938,264
Financial instruments	(2,152,588)	(1,935,200)
Net amount of liabilities in the consolidated balance sheet	17,838	3,064
Credit Suisse First Boston Mortgage Capital LLC
Derivative liabilities:
Net amounts of liabilities presented in the consolidated balance sheet	848,914	691,030
Financial instruments	(845,622)	(690,766)
Net amount of liabilities in the consolidated balance sheet	3,292	264
Bank of America, N.A.
Derivative liabilities:
Net amounts of liabilities presented in the consolidated balance sheet	170,859	170,820
Financial instruments	(170,859)	(170,820)
Deutsche Bank AG
Derivative liabilities:
Net amounts of liabilities presented in the consolidated balance sheet	692,010	741,978
Financial instruments	(692,010)	(741,978)
Royal Bank of Canada
Derivative liabilities:
Net amounts of liabilities presented in the consolidated balance sheet	40,099	35,181
Financial instruments	(40,099)	(35,181)
JP Morgan
Derivative liabilities:
Net amounts of liabilities presented in the consolidated balance sheet	50,875	54,326
Financial instruments	(50,875)	(54,326)
Morgan Stanley Bank
Derivative liabilities:
Net amounts of liabilities presented in the consolidated balance sheet	80,998	77,687
Financial instruments	(71,069)	(77,687)
Net amount of liabilities in the consolidated balance sheet	9,929
Citibank, N.A.
Derivative liabilities:
Net amounts of liabilities presented in the consolidated balance sheet	83,533	14,960
Financial instruments	(82,659)	(14,960)
Net amount of liabilities in the consolidated balance sheet	874
Federal National Mortgage Association
Derivative liabilities:
Net amounts of liabilities presented in the consolidated balance sheet	764
Net amount of liabilities in the consolidated balance sheet	764
BNP Paribas
Derivative liabilities:
Net amounts of liabilities presented in the consolidated balance sheet	199,395	149,675
Financial instruments	(199,395)	(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	796	1,438
Net amount of liabilities in the consolidated balance sheet	796	1,438
Interest rate lock commitments
Derivative liabilities:
Net amounts of liabilities presented in the consolidated balance sheet	2,183	1,169
Net amount of liabilities in the consolidated balance sheet	$ 2,183	$ 1,169