Investments - Summary of Amortized Cost and Fair Value of Investments in Fixed Maturity Securities, Classified as Held-to-Maturity (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Investment [Line Items]
Within one year, Amortized Cost	$ 130,685
After one year through five years, Amortized Cost	328,360
After five years through ten years, Amortized Cost	70,889
After ten years, Amortized Cost	536
Held-to-Maturity, Amortized Cost	599,087	536,583
Within one year, Fair Value	132,090
After one year through five years, Fair Value	343,614
After five years through ten years, Fair Value	76,599
After ten years, Fair Value	615
Total, Fair Value	625,783	561,179
U.S. Agency-Based Mortgage-Backed Securities [Member]
Investment [Line Items]
Held-to-maturity, Amortized Cost	18,012
Held-to-maturity, Fair Value	19,747
Commercial Mortgage-Backed Securities [Member]
Investment [Line Items]
Held-to-maturity, Amortized Cost	47,657
Held-to-maturity, Fair Value	50,028
Asset-Backed Securities [Member]
Investment [Line Items]
Held-to-maturity, Amortized Cost	2,948
Held-to-maturity, Fair Value	$ 3,090