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, 2015	Sep. 30, 2015		Dec. 31, 2014	Sep. 30, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents		$ 576.4	$ 695.2		$ 755.9	$ 742.8
Funds Withheld Receivable		1,664.0	1,710.1
Asset-backed loans		153.1	226.7
Debt		6,285.4	6,310.5	[1],[2]
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Policy loans and other invested assets	[1]	2.5	2.5
Asset-backed loans	[1]	153.1	226.7
Total debt	[1]	80.8	99.1	[2]
Liabilities Related to Investment Contracts, Fair Value Disclosure	[1]	913.5	960.3
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total debt	[1]	6,384.7	6,398.0	[2]
Reported Value Measurement [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Policy loans and other invested assets	[1]	2.5	2.5
Asset-backed loans	[1]	153.1	226.7
Total financial assets		155.6
Total debt	[1]	6,285.4
Liabilities Related to Investment Contracts, Fair Value Disclosure	[1]	1,058.2	1,084.5
Financial Liabilities Fair Value Disclosure		7,343.6
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	[1]	2.5	2.5
Asset-backed loans	[1]	153.1	226.7
Total debt	[1]	6,465.5	6,497.1	[2]
Liabilities Related to Investment Contracts, Fair Value Disclosure	[1]	913.5	960.3
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Funds Withheld Receivable		1,664.0	1,710.1
Assets, Fair Value Disclosure, Recurring		1,683.2	1,780.1
Liabilities, Fair Value Disclosure, Recurring		776.6	781.3
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Funds Withheld Receivable		74.8	84.4
Assets, Fair Value Disclosure, Recurring		74.8	101.3
Liabilities, Fair Value Disclosure, Recurring		768.9	771.5
Total financial assets		155.6	229.2
Financial Liabilities Fair Value Disclosure		994.3	1,059.4
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Funds Withheld Receivable		53.9	45.8
Assets, Fair Value Disclosure, Recurring		53.9	78.6
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		1,535.3	1,579.9
Assets, Fair Value Disclosure, Recurring		1,554.5	1,600.2
Liabilities, Fair Value Disclosure, Recurring		7.7	9.8
Total financial assets		0.0	0.0
Financial Liabilities Fair Value Disclosure		6,384.7	6,398.0
Fair Value, Measurements, Recurring [Member] | Commodity Option [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives		1.3	7.9
Derivative Liability		5.1	5.6
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		1.3	7.9
Derivative Liability		5.1	5.6
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		139.9	142.3
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		139.9	142.3
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] | Interest Rate Contract [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives		10.3	6.4
Fair Value, Measurements, Recurring [Member] | Interest Rate Contract [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] | Interest Rate Contract [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] | Interest Rate Contract [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives		10.3	6.4
Fair Value, Measurements, Recurring [Member] | Foreign Exchange Forward [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability		0.7	1.6
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		0.7	1.6
Fair Value, Measurements, Recurring [Member] | Interest Rate Swap [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability		1.9	2.6
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		1.9	2.6
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		7.6	6.0
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		7.6	6.0
Fair Value, Measurements, Recurring [Member] | Equity [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Securities		0.0	32.8
Fair Value, Measurements, Recurring [Member] | Equity [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Securities		0.0	32.8
Fair Value, Measurements, Recurring [Member] | Other Invested Assets [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other invested assets		0.0	2.8
Fair Value, Measurements, Recurring [Member] | Other Invested Assets [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other invested assets		0.0	0.0
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			229.2
Financial Liabilities Fair Value Disclosure			7,395.0
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		155.6	229.2
Financial Liabilities Fair Value Disclosure		7,379.0	7,457.4
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		629.0	629.2
Corporate Debt Securities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities		0.0	14.1
Corporate Debt Securities [Member] | Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities		0.0	14.1
Corporate Debt Securities [Member] | Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities		0.0	0.0
Other Invested Assets [Member] | Fair Value, Measurements, Recurring [Member] | Other Invested Assets [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other invested assets		0.0
Option Pricing [Member] | Other Invested Assets [Member] | Fair Value, Measurements, Recurring [Member] | Other Invested Assets [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other invested assets			2.8
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		15.4	19.5
Other invested assets		0.0	2.8
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		$ 629.0	$ 629.2

[1] The carrying amounts 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 values of debt set forth above are generally based on quoted or observed market prices.