Debt - Summary of Mortgage Warehouse Borrowings (Detail) - USD ($)	3 Months Ended	9 Months Ended			12 Months Ended
	Dec. 31, 2015	Sep. 30, 2016	Sep. 30, 2015	Sep. 28, 2015	Dec. 31, 2015
Line of Credit Facility [Line Items]
Amount Drawn	$ 183,444,000	$ 91,166,000			$ 183,444,000
Secured Debt [Member]
Line of Credit Facility [Line Items]
Amount Drawn	183,444,000	91,166,000			183,444,000
Facility Amount	245,000,000	205,000,000			245,000,000
Mortgage borrowings outstanding, collateralized amount	201,700,000	119,000,000			201,700,000
Collateralized amount of restricted short-term investments	1,300,000	1,300,000			1,300,000
Secured Debt [Member] | Flagstar [Member]
Line of Credit Facility [Line Items]
Amount Drawn	63,210,000	24,559,000			63,210,000
Facility Amount	75,000,000	$ 55,000,000			$ 75,000,000
Expiration date		30 days written notice	30 days written notice
Expiration Date (in days)		30 days			30 days
Collateral		Mortgage Loans	Mortgage Loans
Secured Debt [Member] | Comerica [Member]
Line of Credit Facility [Line Items]
Amount Drawn	18,009,000	$ 19,183,000			$ 18,009,000
Facility Amount	50,000,000	$ 50,000,000			50,000,000
Expiration Date		Nov. 16, 2016	Nov. 16, 2016
Collateral		Mortgage Loans	Mortgage Loans
Secured Debt [Member] | J.P. Morgan [Member]
Line of Credit Facility [Line Items]
Amount Drawn	102,225,000	$ 47,424,000			102,225,000
Facility Amount	$ 120,000,000	$ 100,000,000			$ 120,000,000
Expiration Date		Sep. 25, 2017	Sep. 26, 2016
Collateral		Mortgage Loans and Pledged Cash	Mortgage Loans and Pledged Cash
Secured Debt [Member] | London Interbank Offered Rate (LIBOR) [Member] | Flagstar [Member]
Line of Credit Facility [Line Items]
Interest rate, spread on variable rate		2.50%			2.50%
Secured Debt [Member] | London Interbank Offered Rate (LIBOR) [Member] | Comerica [Member]
Line of Credit Facility [Line Items]
Interest rate, spread on variable rate		2.25%			2.25%
Secured Debt [Member] | London Interbank Offered Rate (LIBOR) [Member] | J.P. Morgan [Member]
Line of Credit Facility [Line Items]
Interest rate, spread on variable rate		2.375%
Minimum rate for LIBOR				0.25%
Secured Debt [Member] | London Interbank Offered Rate (LIBOR) [Member] | J.P. Morgan [Member] | Minimum [Member]
Line of Credit Facility [Line Items]
Interest rate, spread on variable rate	2.375%			2.50%
Secured Debt [Member] | London Interbank Offered Rate (LIBOR) [Member] | J.P. Morgan [Member] | Maximum [Member]
Line of Credit Facility [Line Items]
Interest rate, spread on variable rate				2.875%