Mortgage Loans - Change in Allowance for Credit Losses in Mortgage Loans (Detail) $ in Thousands	9 Months Ended
	Sep. 30, 2019 USD ($) loan
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Collectively evaluated for impairment, number of loans, beginning balance | loan	449
Collectively evaluated for impairment, number of loans, change in recorded investment | loan	(28)
Collectively evaluated for impairment, number of loans, ending balance | loan	421
Collectively evaluated for impairment, recorded investment, beginning balance	$ 5,128,417
Collectively evaluated for impairment, recorded investment, change in recorded investment	(173,371)
Collectively evaluated for impairment, recorded Investment, ending balance	4,955,046
Collectively evaluated for impairment, valuation allowance, beginning balance	18,607
Collectively evaluated for impairment, valuation allowance, change in allowance	(167)
Collectively evaluated for impairment, valuation allowance, ending balance	$ 18,440
Individually impaired, number of loans, beginning balance | loan	2
Individually impaired, number of loans, change in recorded investment | loan	(1)
Individually impaired, number of loans, ending balance | loan	1
Individually impaired, recorded investment, beginning balance	$ 17,623
Individually impaired, recorded investment, change in recorded investment	(17,129)
Individually impaired, recorded investment, ending balance	494
Individually impaired, valuation allowance, beginning balance	2,726
Individually impaired, valuation allowance, change in allowance	(2,232)
Individually impaired, valuation allowance, ending balance	$ 494
Total, number of loans, beginning balance | loan	451
Total, number of loans, change in recorded investment | loan	(29)
Total, number of loans, ending balance | loan	422
Total, recorded investment, beginning balance	$ 5,146,040
Total, recorded investment, change in recorded investment	(190,500)
Total, recorded investment, ending balance	4,955,540
Total, valuation allowance, beginning balance	21,333
Total, valuation allowance, change in allowance	(2,399)
Total, valuation allowance, ending balance	$ 18,934