Servicing - Additional Information (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013
Servicing
Unpaid principal balance of loans	$ 50,212,264	$ 47,794,561	$ 47,713,739	$ 46,066,660	$ 44,031,890	$ 41,219,196	$ 39,793,733	$ 38,908,865	$ 38,937,027
Loans serviced
Servicing
Unpaid principal balance of loans	50,200,000				44,000,000
Custodial escrow accounts	$ 1,100,000				$ 1,100,000