Investments - Schedule of Amortized Cost and Fair Value on Mortgage-Backed and Asset-Backed Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Schedule Of Available For Sale Securities [Line Items]
Cost or Amortized Cost	$ 650,575	$ 489,846
Fair Value	640,166	483,318
Mortgage-backed securities [Member] | Agency [Member]
Schedule Of Available For Sale Securities [Line Items]
Cost or Amortized Cost	110,724	74,353
Fair Value	109,022	73,854
Mortgage-backed securities [Member] | Non Agency [Member]
Schedule Of Available For Sale Securities [Line Items]
Cost or Amortized Cost	19,408	10,430
Fair Value	19,265	10,183
Asset-backed securities [Member] | Auto Loans [Member]
Schedule Of Available For Sale Securities [Line Items]
Cost or Amortized Cost	37,390	29,883
Fair Value	37,429	29,712
Asset-backed securities [Member] | Credit Card Receivables [Member]
Schedule Of Available For Sale Securities [Line Items]
Cost or Amortized Cost	38,640	32,225
Fair Value	38,568	31,985
Asset-backed securities [Member] | Other Receivables [Member]
Schedule Of Available For Sale Securities [Line Items]
Cost or Amortized Cost	10,517	4,437
Fair Value	10,492	4,426
Mortgage-Backed and Asset-Backed Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Cost or Amortized Cost	216,679	151,328
Fair Value	$ 214,776	$ 150,160