Fair Value Measurements (Summary of Changes in Fair Value of Level 3 Assets Measured on Recurring Basis) (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value at beginning of period			$ 16,538,000
Net realized investment gains (losses)			(1,014,000)
Included in other comprehensive income (loss)			(583,000)
Purchases			8,831,000
Sales			(15,361,000)
Settlements			(53,000)
Fair value at end of period	$ 8,358,000		8,358,000
Corporate Debt Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value at beginning of period	11,358,000
Transfers into Level 3	1,921,000
Transfers out of Level 3	(4,011,000)
Net realized investment gains (losses)	(2,000)		1,000
Included in other comprehensive income (loss)	(40,000)		6,000
Purchases	1,698,000		8,831,000
Sales	(92,000)		(427,000)
Settlements	(2,474,000)		(53,000)
Fair value at end of period	$ 8,358,000		8,358,000
Non-Agency Mortgage-Backed Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value at beginning of period		$ 17,994,000	16,538,000	$ 19,022,000
Transfers into Level 3		0		0
Transfers out of Level 3		0		0
Net realized investment gains (losses)			(1,015,000)
Included in other comprehensive income (loss)		252,000	(589,000)	941,000
Sales			$ (14,934,000)
Settlements		(695,000)		(1,379,000)
Fair value at end of period		$ 17,551,000		17,551,000
Net other-than-temporary impairment losses				$ (1,033,000)