Mortgage Loans - Change in Allowance for Credit Losses in Mortgage Loans (Detail) $ in Thousands	6 Months Ended
	Jun. 30, 2019 USD ($) loan Loans
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Collectively evaluated for impairment, number of loans, beginning balance | Loans	449
Collectively evaluated for impairment, number of loans, change in recorded investment | loan	(9)
Collectively evaluated for impairment, number of loans, ending balance | Loans	440
Collectively evaluated for impairment, recorded investment, beginning balance	$ 5,128,417
Collectively evaluated for impairment, recorded investment, change in recorded investment	(101,423)
Collectively evaluated for impairment, recorded Investment, ending balance	5,026,994
Collectively evaluated for impairment, valuation allowance, beginning balance	18,607
Collectively evaluated for impairment, valuation allowance, change in allowance	(179)
Collectively evaluated for impairment, valuation allowance, ending balance	$ 18,428
Individually impaired, number of loans, beginning balance | Loans	2
Individually impaired, number of loans, change in recorded investment | loan	0
Individually impaired, number of loans, ending balance | Loans	2
Individually impaired, recorded investment, beginning balance	$ 17,623
Individually impaired, recorded investment, change in recorded investment	(8,485)
Individually impaired, recorded investment, ending balance	9,138
Individually impaired, valuation allowance, beginning balance	2,726
Individually impaired, valuation allowance, change in allowance	268
Individually impaired, valuation allowance, ending balance	$ 2,994
Total, number of loans, beginning balance | Loans	451
Total, number of loans, change in recorded investment | loan	(9)
Total, number of loans, ending balance | Loans	442
Total, recorded investment, beginning balance	$ 5,146,040
Total, recorded investment, change in recorded investment	(109,908)
Total, recorded investment, ending balance	5,036,132
Total, valuation allowance, beginning balance	21,333
Total, valuation allowance, change in allowance	89
Total, valuation allowance, ending balance	$ 21,422