Investments - Additional Information (Detail) (USD $)	12 Months Ended			3 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Mar. 31, 2015
Schedule of Investments
Fair value of fixed maturity securities	$ 45,064,900,000	$ 42,344,400,000
Fixed Maturity Securities Other Than Temporary Impairments in Accumulated Other Comprehensive Income Loss	0	0
Commitment to fund private placement fixed maturity securities	15,000,000
Carrying amount of variable interest entity investments	484,100,000
Unfunded commitment to fund Investment Partnerships	12,800,000
Commitment to fund partnership Equity Investments	161,600,000
Other Long-term Investments	591,900,000	566,000,000
Commitments to Fund Special Purpose Entities	0
Amount Funded to Special Purpose Entity Partnerships	nor did we fund any amounts to the partnership	nor did we fund any amounts to the partnership	nor did we fund any amounts to the partnership
Mortgage Loan policy is not to exceed a loan-to-value ratio, percent	75.00%
Loan to Value Ratio Update Frequency	3 years
Mortgage Loan Inspection Frequency	2 years
Mortage Loan Policy Debt Service coverage ratio lower range	125.00%
Mortgage Loans issuance, term years	25
Foreclosed Real Estate Expense	0	0	0
Financing Receivable, Recorder Investment, Nonaccrual status	0	0	0
Number of Changes to Accounting Policy for Estimating Credit Losses on Mortgage Loans	0
Average Recorded Investment of Impaired Mortgage Loans	26,700,000	14,900,000	19,100,000
Interest Income on Mortgage Loans Subsequent to Impairment	1,000,000	800,000	800,000
Committments to Fund Commercial Mortgage Loans	49,300,000
Repurchase agreements - Typical Days outstanding	30
Minimum percent of the fair value of securities loaned or securities purchased under repurchase agreements be maintained as collateral	102.00%
Gross Securities Borrowed	176,500,000	201,600,000
Net Securities Borrowed Reported in Balance Sheet	176,500,000	201,600,000
Outstanding Repurchase Agreements	0	0
Off Balance Sheet Amount
Schedule of Investments
Cash Collateral for Borrowed Securities	58,400,000	76,500,000
Securities Received as Collateral	128,500,000	132,900,000
Commercial Real Estate Portfolio Segment
Schedule of Investments
Unpaid Principal Balance of Impaired Mortgage Loan	14,600,000	14,600,000
Impaired Financing Receivable, Related Allowance	1,500,000	1,500,000
Impaired Financing Receivable, with Related Allowance, Recorded Investment	13,100,000	13,100,000
Special Purpose Entity | Bonds
Schedule of Investments
Fair value of fixed maturity securities	143,900,000	136,200,000
Special Purpose Entity | Partnership Interest
Schedule of Investments
Other Long-term Investments	1,400,000	4,400,000
Partnership Interests In Tax Credit Investments
Schedule of Investments
Carrying amount of variable interest entity investments	289,000,000
Equity Method Investments
Schedule of Investments
Carrying amount of variable interest entity investments	195,100,000
Commercial Real Estate Portfolio Segment
Schedule of Investments
Recorded Investment in Mortgage Loan Modifications Due to Foreclosure	18,100,000	4,300,000	17,300,000
Troubled Debt Restructuring Write-down	3,000,000
External Credit Rating, Investment Grade
Schedule of Investments
Fair value of fixed maturity securities	41,539,700,000
Accumulated Gross Unrealized Gain on Securities	6,238,000,000
Accumulated Gross Unrealized Loss on Securities	47,500,000
Percent of Fixed Maturity Securities in Unrealized Loss Position	38.10%
Number of fixed maturity securities that were in an unrealized loss position	75
Number of fixed maturity securities in an unrealized loss position continuously for over one year	31
External Credit Rating, Non Investment Grade
Schedule of Investments
Fair value of fixed maturity securities	3,525,200,000
Accumulated Gross Unrealized Gain on Securities	148,300,000
Accumulated Gross Unrealized Loss on Securities	77,300,000
Percent of Fixed Maturity Securities in Unrealized Loss Position	61.90%
Number of fixed maturity securities that were in an unrealized loss position	68
Number of fixed maturity securities in an unrealized loss position continuously for over one year	15
Subsequent Event
Schedule of Investments
Federal Home Loan Bank Stock				17,700,000
Purchase Commitment
Schedule of Investments
Federal Home Loan Bank Stock	$ 12,500,000