Investments - Schedule of Amortized Cost and Fair Value on Mortgage-Backed and Asset-Backed Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 931,174	$ 815,647
Fair Value	913,131	819,861
Mortgage-backed and asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	330,939	319,377
Fair Value	325,697	319,937
Agency | Mortgage-backed Securities:
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	159,583	153,937
Fair Value	156,281	153,758
Non-agency | Mortgage-backed Securities:
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	57,231	54,231
Fair Value	55,262	54,666
Auto loan receivables | Asset-backed Securities:
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	68,860	68,188
Fair Value	68,908	68,440
Credit card receivables | Asset-backed Securities:
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	7,850	7,878
Fair Value	7,855	7,891
Other receivables | Asset-backed Securities:
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	37,415	35,143
Fair Value	$ 37,391	$ 35,182