Mortgage Loans - Change in Allowance for Credit Losses in Mortgage Loans (Detail) $ in Thousands	3 Months Ended
	Mar. 31, 2019 USD ($) loan Loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for credit losses, collectively evaluated for impairment, number of loans, beginning balance | Loans	449
Allowance for credit losses, collectively evaluated for impairment, number of loans, change in recorded investment | loan	(13)
Allowance for credit losses, collectively evaluated for impairment, number of loans, ending balance | Loans	436
Allowance for credit losses, collectively evaluated for impairment, recorded investment, beginning balance	$ 5,128,417
Allowance for credit losses, collectively evaluated for impairment, recorded investment, change in recorded investment	(164,173)
Allowance for credit losses, collectively evaluated for impairment, recorded Investment, ending balance	4,964,244
Allowance for credit losses, collectively evaluated for impairment, valuation allowance, beginning balance	18,607
Allowance for credit losses, collectively evaluated for impairment, valuation allowance, change in allowance	(455)
Allowance for credit losses, collectively evaluated for impairment, valuation allowance, ending balance	$ 18,152
Allowance for credit losses, individually impaired, number of loans, beginning balance | Loans	2
Allowance for credit losses, individually impaired, number of loans, change in recorded investment | loan	1
Allowance for credit losses, individually impaired, number of loans, ending balance | Loans	3
Allowance for credit losses, individually impaired, recorded investment, beginning balance	$ 17,623
Allowance for credit losses, individually impaired, recorded investment, change in recorded investment	(339)
Allowance for credit losses, individually impaired, recorded investment, ending balance	17,284
Allowance for credit losses, individually impaired, valuation allowance, beginning balance	2,726
Allowance for credit losses, individually impaired, valuation allowance, ending balance	$ 2,726
Allowance for credit losses, number of loans, beginning balance | Loans	451
Allowance for credit losses, number of loans, change in recorded investment | loan	(12)
Allowance for credit losses, number of loans, ending balance | Loans	439
Allowance for credit losses, recorded investment, beginning balance	$ 5,146,040
Allowance for credit losses, recorded investment, change in recorded investment	(164,512)
Allowance for credit losses, recorded investment, ending balance	4,981,528
Allowance for credit losses, valuation allowance, beginning balance	21,333
Allowance for credit losses, valuation allowance, change in allowance	(455)
Allowance for credit losses, valuation allowance, ending balance	$ 20,878