Investments Investment Income (Detail) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Net Investment Income
Investment Income, Interest and Dividend		$ 579.6	$ 635.4	$ 1,176.3	$ 1,242.4
Less Investment Expenses		7.6	7.3	16.1	16.3
Less Investment Income on PFA Assets		3.0	3.2	6.2	6.5
Net Investment Income		569.0	624.9	1,154.0	1,219.6
Fixed Maturity Securities
Net Investment Income
Investment Income, Interest and Dividend		542.5	564.7	1,082.3	1,109.6
Derivatives
Net Investment Income
Investment Income, Interest and Dividend		20.7	18.0	40.9	35.9
Mortgage Loans
Net Investment Income
Investment Income, Interest and Dividend		28.0	24.9	57.0	51.6
Policy Loans
Net Investment Income
Investment Income, Interest and Dividend		5.0	4.8	9.8	9.5
Perpetual Preferred Securities
Net Investment Income
Investment Income, Interest and Dividend	[1]	10.6	0.4	(6.5)	4.5
Private Equity Partnerships
Net Investment Income
Investment Income, Interest and Dividend	[2]	(31.3)	12.6	(20.9)	14.8
Other Long-term Investments
Net Investment Income
Investment Income, Interest and Dividend		1.7	3.3	4.9	2.8
Short-term Investments
Net Investment Income
Investment Income, Interest and Dividend		$ 2.4	$ 6.7	$ 8.8	$ 13.7

[1] The net unrealized gain (loss) recognized in net investment income for the three and six months ended June 30, 2020 related to perpetual preferred securities still held at June 30, 2020 was $10.0 million and $(7.9) million, respectively. The net unrealized gain (loss) recognized in net investment income for the three and six months ended June 30, 2019 related to perpetual preferred securities still held at June 30, 2019 was $(0.1) million and $3.0 million.

[2] The net unrealized loss recognized in net investment income for the three and six months ended June 30, 2020 related to private equity partnerships still held at June 30, 2020 was $41.0 million and $35.6 million. The net unrealized gain recognized in net investment income for the three and six months ended June 30, 2019 related to private equity partnerships still held at June 30, 2019 was $6.3 million and $3.6 million. See Note 3 for further discussion of private equity partnerships.