Fair Value of Financial Instruments Fair Value of Financial Instruments - Assets and Liabilities Carrying at Fair Value Measured on a Recurring Basis (Details) - USD ($) $ in Millions		Dec. 31, 2016	Sep. 30, 2016		Dec. 31, 2015	Sep. 30, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents		$ 348.8	$ 497.3		$ 566.5	$ 661.2
Funds Withheld Receivable		1,609.0	1,650.4
Debt		5,459.0	5,430.9	[1],[2]
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Policy loans and other invested assets		8.3
Asset-backed loans	[1]	13.7	35.0
Total debt	[1],[2]	23.3	29.1
Liabilities Related to Investment Contracts, Fair Value Disclosure	[1]	897.7	922.9
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total debt	[1],[2]	5,677.7	5,700.1
Policy Loans [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Policy loans and other invested assets			8.5
Reported Value Measurement [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Policy loans and other invested assets		8.3
Asset-backed loans	[1]	13.7	35.0
Total financial assets		22.0
Total debt	[1],[2]	5,459.0
Liabilities Related to Investment Contracts, Fair Value Disclosure	[1]	966.2	988.3
Financial Liabilities Fair Value Disclosure		6,425.2
Reported Value Measurement [Member] | Policy Loans [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Policy loans and other invested assets			8.5
Estimate of Fair Value Measurement [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Policy loans and other invested assets		8.3
Asset-backed loans	[1]	13.7	35.0
Total debt	[1],[2]	5,701.0	5,729.2
Liabilities Related to Investment Contracts, Fair Value Disclosure	[1]	897.7	922.9
Estimate of Fair Value Measurement [Member] | Policy Loans [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Policy loans and other invested assets			8.5
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Funds Withheld Receivable		1,532.8	1,535.1
Assets, Fair Value Disclosure, Recurring		1,566.9	1,561.0
Liabilities, Fair Value Disclosure, Recurring		757.1	766.2
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Funds Withheld Receivable		46.4	78.1
Assets, Fair Value Disclosure, Recurring		46.4	78.1
Liabilities, Fair Value Disclosure, Recurring		755.7	763.0
Total financial assets		22.0	43.5
Financial Liabilities Fair Value Disclosure		921.0	952.0
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Funds Withheld Receivable		79.6	69.9
Assets, Fair Value Disclosure, Recurring		79.6	69.9
Liabilities, Fair Value Disclosure, Recurring		0.0	0.0
Total financial assets		0.0	0.0
Financial Liabilities Fair Value Disclosure		0.0	0.0
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Funds Withheld Receivable		1,406.8	1,387.1
Assets, Fair Value Disclosure, Recurring		1,440.9	1,413.0
Liabilities, Fair Value Disclosure, Recurring		1.4	3.2
Total financial assets		0.0	0.0
Financial Liabilities Fair Value Disclosure		5,677.7	5,700.1
Fair Value, Measurements, Recurring [Member] | Commodity Option [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives		3.3	2.9
Derivative Liability		0.2	0.1
Fair Value, Measurements, Recurring [Member] | Commodity Option [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives		3.3	2.9
Derivative Liability		0.2	0.1
Fair Value, Measurements, Recurring [Member] | Fixed Index Annuity [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability		121.2	131.2
Fair Value, Measurements, Recurring [Member] | Fixed Index Annuity [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability		121.2	131.2
Fair Value, Measurements, Recurring [Member] | Front Street Future Policyholder Benefits Reserve [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability			0.0
Fair Value, Measurements, Recurring [Member] | Foreign Exchange Forward [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability		0.5	2.0
Fair Value, Measurements, Recurring [Member] | Foreign Exchange Forward [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability		0.5	2.0
Fair Value, Measurements, Recurring [Member] | Call options [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives		8.3	5.9
Fair Value, Measurements, Recurring [Member] | Call options [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives		0.0	0.0
Fair Value, Measurements, Recurring [Member] | Call options [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives		0.0	0.0
Fair Value, Measurements, Recurring [Member] | Call options [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives		8.3	5.9
Fair Value, Measurements, Recurring [Member] | Interest Rate Swap [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability		0.7	1.1
Fair Value, Measurements, Recurring [Member] | Interest Rate Swap [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability		0.7	1.1
Fair Value, Measurements, Recurring [Member] | Foreign Exchange Contract [Member] | Foreign Exchange Forward [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives		10.3	5.8
Fair Value, Measurements, Recurring [Member] | Foreign Exchange Contract [Member] | Foreign Exchange Forward [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives			0.0
Fair Value, Measurements, Recurring [Member] | Foreign Exchange Contract [Member] | Foreign Exchange Forward [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives		10.3	5.8
Fair Value, Measurements, Recurring [Member] | Reported Value Measurement [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total financial assets			43.5
Financial Liabilities Fair Value Disclosure			6,419.2
Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value Measurement [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total financial assets		22.0	43.5
Financial Liabilities Fair Value Disclosure		6,598.7	6,652.1
Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value Measurement [Member] | Front Street Future Policyholder Benefits Reserve [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability		634.5	631.8
Discounted Cash Flow [Member] | Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Funds Withheld Receivable		5.3	5.4
Discounted Cash Flow [Member] | Fair Value, Measurements, Recurring [Member] | Front Street Future Policyholder Benefits Reserve [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability		$ 634.5	$ 631.8

[1] The carrying value of cash and cash equivalents, trade receivables, accounts payable and accrued investment income approximate fair value due to their short duration and, accordingly, they are not presented in the tables above.

[2] The fair value of debt set forth above is generally based on quoted or observed market prices.