Condensed Consolidating Financial Information - Statements of Cash Flows (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014
Condensed Financial Statements, Captions [Line Items]
Net cash provided by operating activities	$ 2,686.4	$ 2,916.2
Proceeds from the sale, maturity, disposal or redemption of:
Fixed maturities	8,040.5	9,192.3
Equity securities, available-for-sale	38.2	63.7
Mortgage loans on real estate	950.6	937.6
Limited partnerships/corporations	198.3	137.6
Acquisition of:
Fixed maturities	(9,699.4)	(9,172.3)
Equity securities, available-for-sale	(114.1)	(18.3)
Mortgage loans on real estate	(1,883.4)	(1,574.8)
Limited partnerships/corporations	(332.5)	(261.3)
Short-term investments, net	139.9	(124.5)
Policy loans, net	76.8	42.8
Derivatives, net	297.9	(670.7)
Other investments, net	18.7	38.5
Sales from consolidated investment entities	4,087.9	2,558.5
Purchases within consolidated investment entities	(6,056.5)	(4,292.6)
Maturity of intercompany loans with maturities more than three months	0.0	0.0
Net maturity of short-term intercompany loans	0.0	0.0
Return of capital contributions from subsidiaries	0.0	0.0
Capital contributions to subsidiaries		0.0
Collateral received (delivered), net	530.5	116.8
Purchases of fixed assets, net	(38.3)	(26.5)
Net cash used in investing activities	(3,744.9)	(3,053.2)
Cash Flows from Financing Activities:
Deposits received for investment contracts	5,635.4	5,681.5
Maturities and withdrawals from investment contracts	(5,018.2)	(7,332.9)
Repayment of debt with maturities of more than three months	(31.2)	0.0
Debt issuance costs	(6.8)	(16.8)
Intercompany loans with maturities of more than three months	0.0	0.0
Net (repayments of) proceeds from short-term intercompany loans	0.0	0.0
Return of capital contributions to parent	0.0	0.0
Contributions of capital from parent		0.0
Borrowings of consolidated investment entities	1,412.6	340.5
Repayments of borrowings of consolidated investment entities	(444.4)	(66.6)
Contributions from (distributions to) participants in consolidated investment entities, net	841.4	1,235.9
Excess tax benefits on share-based compensation	1.7	0.0
Share-based compensation	(4.4)	(14.8)
Common stock acquired - Share repurchase	(1,340.5)	(614.4)
Dividends paid	(6.9)	(7.7)
Net cash provided by (used in) financing activities	1,038.7	(795.3)
Net decrease in cash and cash equivalents	(19.8)	(932.3)
Cash and cash equivalents, beginning of year	2,530.9	2,840.8
Cash and cash equivalents, end of year	2,511.1	1,908.5
Parent Issuer
Condensed Financial Statements, Captions [Line Items]
Net cash provided by operating activities	32.7	(163.3)
Proceeds from the sale, maturity, disposal or redemption of:
Fixed maturities	0.0	0.0
Equity securities, available-for-sale	19.4	13.0
Mortgage loans on real estate	0.0	0.0
Limited partnerships/corporations	0.0	0.0
Acquisition of:
Fixed maturities	0.0	0.0
Equity securities, available-for-sale	(23.5)	(15.6)
Mortgage loans on real estate	0.0	0.0
Limited partnerships/corporations	0.0	0.0
Short-term investments, net	(212.0)	0.0
Policy loans, net	0.0	0.0
Derivatives, net	(4.0)	1.4
Other investments, net	0.0	0.0
Sales from consolidated investment entities	0.0	0.0
Purchases within consolidated investment entities	0.0	0.0
Maturity of intercompany loans with maturities more than three months	0.7	0.8
Net maturity of short-term intercompany loans	(96.3)	4.2
Return of capital contributions from subsidiaries	1,281.0	797.0
Capital contributions to subsidiaries		(150.0)
Collateral received (delivered), net	0.0	0.0
Purchases of fixed assets, net	0.0	0.0
Net cash used in investing activities	965.3	650.8
Cash Flows from Financing Activities:
Deposits received for investment contracts	0.0	0.0
Maturities and withdrawals from investment contracts	0.0	0.0
Repayment of debt with maturities of more than three months	0.0
Debt issuance costs	(6.8)	(16.8)
Intercompany loans with maturities of more than three months	0.0	0.0
Net (repayments of) proceeds from short-term intercompany loans	0.0	0.0
Return of capital contributions to parent	0.0	0.0
Contributions of capital from parent		0.0
Borrowings of consolidated investment entities	0.0	0.0
Repayments of borrowings of consolidated investment entities	0.0	0.0
Contributions from (distributions to) participants in consolidated investment entities, net	0.0	0.0
Excess tax benefits on share-based compensation	0.0
Share-based compensation	(4.4)	(14.8)
Common stock acquired - Share repurchase	(1,340.5)	(614.4)
Dividends paid	(6.9)	(7.7)
Net cash provided by (used in) financing activities	(1,358.6)	(653.7)
Net decrease in cash and cash equivalents	(360.6)	(166.2)
Cash and cash equivalents, beginning of year	682.1	640.2
Cash and cash equivalents, end of year	321.5	474.0
Subsidiary Guarantor
Condensed Financial Statements, Captions [Line Items]
Net cash provided by operating activities	67.5	97.2
Proceeds from the sale, maturity, disposal or redemption of:
Fixed maturities	0.0	0.0
Equity securities, available-for-sale	0.0	13.0
Mortgage loans on real estate	0.0	0.0
Limited partnerships/corporations	0.0	0.0
Acquisition of:
Fixed maturities	0.0	0.0
Equity securities, available-for-sale	0.0	0.0
Mortgage loans on real estate	0.0	0.0
Limited partnerships/corporations	0.0	0.0
Short-term investments, net	0.0	0.0
Policy loans, net	0.0	0.0
Derivatives, net	0.0	0.0
Other investments, net	13.7	0.8
Sales from consolidated investment entities	0.0	0.0
Purchases within consolidated investment entities	0.0	0.0
Maturity of intercompany loans with maturities more than three months	0.0	0.0
Net maturity of short-term intercompany loans	0.0	0.0
Return of capital contributions from subsidiaries	1,197.7	690.0
Capital contributions to subsidiaries		(171.0)
Collateral received (delivered), net	0.0	0.0
Purchases of fixed assets, net	0.0	0.0
Net cash used in investing activities	1,211.4	532.8
Cash Flows from Financing Activities:
Deposits received for investment contracts	0.0	0.0
Maturities and withdrawals from investment contracts	0.0	0.0
Repayment of debt with maturities of more than three months	(31.2)
Debt issuance costs	0.0	0.0
Intercompany loans with maturities of more than three months	0.0	0.0
Net (repayments of) proceeds from short-term intercompany loans	33.7	62.6
Return of capital contributions to parent	(1,281.0)	(690.0)
Contributions of capital from parent		0.0
Borrowings of consolidated investment entities	0.0	0.0
Repayments of borrowings of consolidated investment entities	0.0	0.0
Contributions from (distributions to) participants in consolidated investment entities, net	0.0	0.0
Excess tax benefits on share-based compensation	0.0
Share-based compensation	0.0	0.0
Common stock acquired - Share repurchase	0.0	0.0
Dividends paid	0.0	0.0
Net cash provided by (used in) financing activities	(1,278.5)	(627.4)
Net decrease in cash and cash equivalents	0.4	2.6
Cash and cash equivalents, beginning of year	1.6	1.1
Cash and cash equivalents, end of year	2.0	3.7
Non-Guarantor Subsidiaries
Condensed Financial Statements, Captions [Line Items]
Net cash provided by operating activities	2,916.5	3,108.3
Proceeds from the sale, maturity, disposal or redemption of:
Fixed maturities	8,040.5	9,192.3
Equity securities, available-for-sale	18.8	37.7
Mortgage loans on real estate	950.6	937.6
Limited partnerships/corporations	198.3	137.6
Acquisition of:
Fixed maturities	(9,699.4)	(9,172.3)
Equity securities, available-for-sale	(90.6)	(2.7)
Mortgage loans on real estate	(1,883.4)	(1,574.8)
Limited partnerships/corporations	(332.5)	(261.3)
Short-term investments, net	351.9	(124.5)
Policy loans, net	76.8	42.8
Derivatives, net	301.9	(672.1)
Other investments, net	5.0	37.7
Sales from consolidated investment entities	4,087.9	2,558.5
Purchases within consolidated investment entities	(6,056.5)	(4,292.6)
Maturity of intercompany loans with maturities more than three months	0.0	0.0
Net maturity of short-term intercompany loans	0.0	0.0
Return of capital contributions from subsidiaries	0.0	0.0
Capital contributions to subsidiaries		0.0
Collateral received (delivered), net	530.5	116.8
Purchases of fixed assets, net	(38.3)	(26.5)
Net cash used in investing activities	(3,538.5)	(3,065.8)
Cash Flows from Financing Activities:
Deposits received for investment contracts	5,635.4	5,681.5
Maturities and withdrawals from investment contracts	(5,018.2)	(7,332.9)
Repayment of debt with maturities of more than three months	0.0
Debt issuance costs	0.0	0.0
Intercompany loans with maturities of more than three months	(0.7)	(0.8)
Net (repayments of) proceeds from short-term intercompany loans	62.6	(66.8)
Return of capital contributions to parent	(1,528.0)	(923.0)
Contributions of capital from parent		321.0
Borrowings of consolidated investment entities	1,412.6	340.5
Repayments of borrowings of consolidated investment entities	(444.4)	(66.6)
Contributions from (distributions to) participants in consolidated investment entities, net	841.4	1,235.9
Excess tax benefits on share-based compensation	1.7
Share-based compensation	0.0	0.0
Common stock acquired - Share repurchase	0.0	0.0
Dividends paid	0.0	0.0
Net cash provided by (used in) financing activities	962.4	(811.2)
Net decrease in cash and cash equivalents	340.4	(768.7)
Cash and cash equivalents, beginning of year	1,847.2	2,199.5
Cash and cash equivalents, end of year	2,187.6	1,430.8
Consolidating Adjustments
Condensed Financial Statements, Captions [Line Items]
Net cash provided by operating activities	(330.3)	(126.0)
Proceeds from the sale, maturity, disposal or redemption of:
Fixed maturities	0.0	0.0
Equity securities, available-for-sale	0.0	0.0
Mortgage loans on real estate	0.0	0.0
Limited partnerships/corporations	0.0	0.0
Acquisition of:
Fixed maturities	0.0	0.0
Equity securities, available-for-sale	0.0	0.0
Mortgage loans on real estate	0.0	0.0
Limited partnerships/corporations	0.0	0.0
Short-term investments, net	0.0	0.0
Policy loans, net	0.0	0.0
Derivatives, net	0.0	0.0
Other investments, net	0.0	0.0
Sales from consolidated investment entities	0.0	0.0
Purchases within consolidated investment entities	0.0	0.0
Maturity of intercompany loans with maturities more than three months	(0.7)	(0.8)
Net maturity of short-term intercompany loans	96.3	(4.2)
Return of capital contributions from subsidiaries	(2,478.7)	(1,487.0)
Capital contributions to subsidiaries		321.0
Collateral received (delivered), net	0.0	0.0
Purchases of fixed assets, net	0.0	0.0
Net cash used in investing activities	(2,383.1)	(1,171.0)
Cash Flows from Financing Activities:
Deposits received for investment contracts	0.0	0.0
Maturities and withdrawals from investment contracts	0.0	0.0
Repayment of debt with maturities of more than three months	0.0
Debt issuance costs	0.0	0.0
Intercompany loans with maturities of more than three months	0.7	0.8
Net (repayments of) proceeds from short-term intercompany loans	(96.3)	4.2
Return of capital contributions to parent	2,809.0	1,613.0
Contributions of capital from parent		(321.0)
Borrowings of consolidated investment entities	0.0	0.0
Repayments of borrowings of consolidated investment entities	0.0	0.0
Contributions from (distributions to) participants in consolidated investment entities, net	0.0	0.0
Excess tax benefits on share-based compensation	0.0
Share-based compensation	0.0	0.0
Common stock acquired - Share repurchase	0.0	0.0
Dividends paid	0.0	0.0
Net cash provided by (used in) financing activities	2,713.4	1,297.0
Net decrease in cash and cash equivalents	0.0	0.0
Cash and cash equivalents, beginning of year	0.0	0.0
Cash and cash equivalents, end of year	$ 0.0	$ 0.0