Investments - Schedule of Amortized Cost and Fair Value on Mortgage-Backed and Asset-Backed Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 851,826	$ 828,336
Fair Value	867,249	855,284
Mortgage-Backed and Asset-Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	314,936	304,285
Fair Value	323,382	311,079
Agency [Member] | Mortgage-backed securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	154,253	143,723
Fair Value	159,262	144,729
Non Agency [Member] | Mortgage-backed securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	69,919	71,140
Fair Value	74,088	75,896
Auto Loan Receivables [Member] | Asset-backed securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	42,796	42,767
Fair Value	42,658	43,127
Credit Card Receivables [Member] | Asset-backed securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	20,677	21,145
Fair Value	20,594	21,487
Other Receivables [Member] | Asset-backed securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	27,291	25,510
Fair Value	$ 26,780	$ 25,840