Mortgage Loans - Change in Allowance for Credit Losses in Mortgage Loans (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($) loan	Dec. 31, 2018 USD ($) loan
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Collectively evaluated for impairment, number of loans, beginning balance | loan	449	451
Collectively evaluated for impairment, number of loans, change in recorded investment | loan	(28)	(2)
Collectively evaluated for impairment, number of loans, ending balance | loan	421	449
Collectively evaluated for impairment, recorded Investment, beginning balance	$ 5,128,417	$ 4,762,315
Collectively evaluated for impairment, recorded investment, change in recorded investment	(12,733)	366,102
Collectively evaluated for impairment, recorded investment, ending balance	5,115,684	5,128,417
Collectively evaluated for impairment, valuation allowance, beginning balance	18,607	16,041
Collectively evaluated for impairment, valuation allowance, change in allowance	60	2,566
Collectively evaluated for impairment, valuation allowance, ending balance	$ 18,667	$ 18,607
Individually impaired, number of loans, beginning balance | loan	2	3
Individually impaired, number of loans, change in recorded investment | loan	(1)	(1)
Individually impaired, number of loans, ending balance | loan	1	2
Individually impaired, recorded investment, beginning balance	$ 17,623	$ 6,550
Individually impaired, recorded investment, change in recorded investment	(17,130)	11,073
Individually impaired, recorded investment, ending balance	493	17,623
Individually impaired, valuation allowance, beginning balance	2,726	2,825
Individually impaired, valuation allowance, change in allowance	(2,233)	(99)
Individually impaired, valuation allowance, ending balance	$ 493	$ 2,726
Total, number of loans, beginning balance | loan	451	454
Total, number of loans, change in recorded investment | loan	(29)	(3)
Total, number of loans, ending balance | loan	422	451
Total, recorded investment, beginning balance	$ 5,146,040	$ 4,768,865
Total, recorded investment, change in recorded investment	(29,863)	377,175
Total, recorded investment, ending balance	5,116,177	5,146,040
Total, valuation allowance, beginning balance	21,333	18,866
Total, valuation allowance, change in allowance	(2,173)	2,467
Total, valuation allowance, ending balance	$ 19,160	$ 21,333