Derivative Financial Instruments - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Jun. 30, 2016	Dec. 31, 2015	Jun. 30, 2015	Dec. 31, 2014
Derivative
Derivative Assets	$ 37,100,000		$ 37,100,000			$ 49,800,000
Cash Collateral from Counterparties	28,800,000		28,800,000			36,400,000
Carrying Value of Fixed Maturity Securities Posted as Collateral to Counterparties	39,000,000		39,000,000			27,300,000
Cash Collateral to Counterparties	0		0			0
Aggregate Fair Value of all Derivative Instruments with Credit Risk-related Contingent Features in a Liability Position	55,700,000		55,700,000			50,200,000
Notional Amount of Derivatives	1,086,900,000	$ 1,700,200,000	1,086,900,000	$ 1,700,200,000	$ 1,440,500,000	1,470,300,000	$ 1,638,700,000	$ 1,687,400,000
Material Ineffectiveness on Cash Flow Hedges	0	0	0	0
Gain from Components Excluded from Assessment of Cash Flow Hedge Effectiveness	0	0	0	0
Approximate Amount of Net Deferred Gains on Derivative Instruments Expected to be Amortized During the Next Twelve Months			56,800,000
Notional Amount Of Derivatives Terminations	357,100,000	5,500,000	386,900,000	83,200,000
Loss on Derivative Contract Termination			1,200,000
Material Ineffectiveness on Fair Value Hedges	0	0	0	0
Component of Derivative Gain (Loss) Excluded from the Assessment of Hedge Effectiveness	0	0	0	0
Discontinued Hedge Accounting Due to Instrument No Longer Qualifying as Fair Value Hedge			0
Derivative, Amount of Hedged Item	3,500,000		3,500,000
Interest Rate Swaps
Derivative
Derivative Assets	3,200,000		3,200,000			2,400,000
Receive Variable/Pay Fixed
Derivative
Notional Amount of Derivatives	126,000,000		126,000,000			150,000,000
Change in Unrealized Gain (Loss) on Fair Value Hedging Instruments	1,700,000	700,000	4,600,000	2,800,000
Receive Fixed/Pay Variable
Derivative
Notional Amount of Derivatives	250,000,000		250,000,000			600,000,000
Change in Unrealized Gain (Loss) on Fair Value Hedging Instruments	3,100,000	(4,700,000)	(3,500,000)	(5,100,000)
Cash Flow Hedging | Interest Rate Swaps
Derivative
Notional Amount of Derivatives	415,000,000		415,000,000			427,900,000
Fair Value Hedging | Receive Fixed/Pay Variable
Derivative
Notional Amount of Derivatives Matured			350,000,000
Amount of Hedged Item, Matured			350,000,000
Not Designated as Hedging Instrument | Interest Rate Swaps
Derivative
Notional Amount of Derivatives	222,400,000		222,400,000			222,400,000
Not Designated as Hedging Instrument | Receive Variable/Pay Fixed
Derivative
Notional Amount of Derivatives	3,500,000		3,500,000
Not Designated as Hedging Instrument | Credit Default Swaps
Derivative
Notional Amount of Derivatives	70,000,000		70,000,000			70,000,000
Receive Variable/Pay Fixed | Swaps
Derivative
Notional Amount of Derivatives	129,500,000	150,000,000	129,500,000	150,000,000	$ 126,000,000	$ 150,000,000	$ 150,000,000	$ 150,000,000
Notional Amount Of Derivatives Terminations	0	$ 0	24,000,000	$ 0
Total Amount
Derivative
Cash Collateral from Counterparties	34,000,000		34,000,000
Credit Exposure
Derivative
Derivative Assets	$ 100,000.0		$ 100,000.0