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		3 Months Ended			6 Months Ended
		Mar. 31, 2017		Mar. 31, 2016	Mar. 31, 2017		Mar. 31, 2016	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]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value		$ 49.1		$ 64.5	$ 49.1		$ 64.5	$ 46.4	$ 78.1		$ 65.1	$ 91.6
Cash and cash equivalents		320.3		457.5	320.3		457.5		497.3			661.2
Funds Withheld Receivable		1,634.2			1,634.2				1,650.4
Debt		5,623.9			5,623.9				5,430.9	[1],[2]
Total Gains (Losses) Included in Earnings				(0.3)	(1.3)		0.3
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases		6.1		3.1	9.1		8.1
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales		(1.0)		(3.4)	(7.3)		(30.0)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements							(5.5)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers, Net		(2.4)	[3]		(29.5)	[4]
Fair Value, Measurement with Unobservable Inputs Reconciliations, Recurring Basis, Liability Value		782.2		790.5	782.2		790.5	755.7	763.0		768.9	771.5
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Earnings		20.9		20.5	(1.7)		14.4
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Other Comprehensive Income (Loss)		0.0		0.0	0.0		0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Purchases		0.0		0.0	0.0		0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Sales		0.0		0.0	0.0		0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Settlements		5.6		1.1	20.9		(4.6)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Transfers, Net		0.0	[3]	0.0	0.0	[4]	0.0
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Policy loans and other invested assets	[1]	8.3			8.3				8.5
Asset-backed loans	[1]	6.3			6.3				35.0
Total debt	[1],[2]	10.1			10.1				29.1
Liabilities Related to Investment Contracts, Fair Value Disclosure	[1]	882.6			882.6				922.9
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total debt	[1],[2]	5,867.8			5,867.8				5,700.1
Reported Value Measurement [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Policy loans and other invested assets	[1]	8.3			8.3				8.5
Asset-backed loans	[1]	6.3			6.3				35.0
Total financial assets		14.6			14.6
Total debt	[1],[2]	5,623.9			5,623.9
Liabilities Related to Investment Contracts, Fair Value Disclosure	[1]	946.0			946.0				988.3
Financial Liabilities Fair Value Disclosure		6,569.9			6,569.9
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]	8.3			8.3				8.5
Asset-backed loans	[1]	6.3			6.3				35.0
Total debt	[1],[2]	5,877.9			5,877.9				5,729.2
Liabilities Related to Investment Contracts, Fair Value Disclosure	[1]	882.6			882.6				922.9
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Funds Withheld Receivable		1,543.4			1,543.4				1,535.1
Assets, Fair Value Disclosure, Recurring		1,576.1			1,576.1				1,561.0
Liabilities, Fair Value Disclosure, Recurring		784.4			784.4				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		49.1			49.1				78.1
Assets, Fair Value Disclosure, Recurring		49.1			49.1				78.1
Liabilities, Fair Value Disclosure, Recurring		782.2			782.2				763.0
Total financial assets		14.6			14.6				43.5
Financial Liabilities Fair Value Disclosure		892.7			892.7				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		80.5			80.5				69.9
Assets, Fair Value Disclosure, Recurring		80.5			80.5				69.9
Liabilities, Fair Value Disclosure, Recurring		0.0			0.0				0.0
Total financial assets		0.0			0.0				0.0
Financial Liabilities Fair Value Disclosure		0.0			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		1,413.8			1,413.8				1,387.1
Assets, Fair Value Disclosure, Recurring		1,446.5			1,446.5				1,413.0
Liabilities, Fair Value Disclosure, Recurring		2.2			2.2				3.2
Total financial assets		0.0			0.0				0.0
Financial Liabilities Fair Value Disclosure		5,867.8			5,867.8				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		4.3			4.3				2.9
Derivative Liability		0.1			0.1				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		4.3			4.3				2.9
Derivative Liability		0.1			0.1				0.1
Fair Value, Measurements, Recurring [Member] | Fixed Index Annuity [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability		121.0			121.0				131.2
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		121.0			121.0				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		2.1			2.1				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		2.1			2.1				2.0
Fair Value, Measurements, Recurring [Member] | Call options [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives		10.5			10.5				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				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				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		10.5			10.5				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									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									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		4.6			4.6				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		4.6			4.6				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		14.6			14.6				43.5
Financial Liabilities Fair Value Disclosure		6,760.5			6,760.5				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		661.2			661.2				631.8
Funds withheld for reinsurance liability [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value		49.1		64.5	49.1		64.5	46.4	78.1		65.1
Total Gains (Losses) Included in Earnings		0.0		(0.3)	(1.3)
Total Gains (Losses) Included in AOCI		0.0		0.0	0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases		6.1		3.1	9.1
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales		(1.0)		(3.4)	(7.3)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements		0.0		0.0	0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers, Net		(2.4)	[3]	0.0	(29.5)	[4]
Corporate Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value												14.1
Total Gains (Losses) Included in Earnings							(0.5)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales							(13.6)
Other Invested Assets [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value				0.0			0.0					2.8
Total Gains (Losses) Included in Earnings							2.7
Total Gains (Losses) Included in AOCI							0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases							0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales							0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements							(5.5)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers, Net							0.0
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		661.2			661.2				631.8
Fixed Index Annuity [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliations, Recurring Basis, Liability Value		121.0		136.5	121.0		136.5	121.2	131.2		139.9	142.3
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Earnings		(0.2)		(3.4)	(10.2)		(5.8)
Front Street Future Policyholder Benefits Reserve [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliations, Recurring Basis, Liability Value		661.2		654.0	661.2		654.0	$ 634.5	$ 631.8		$ 629.0	$ 629.2
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Earnings		21.1		23.9	8.5		20.2
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Settlements		$ 5.6		$ 1.1	$ 20.9		$ (4.6)

[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.

[3] (a) During the three months ended March 31, 2017, the net transfer out of Level 3 was exclusively to Level 2.

[4] (a) During the six months ended March 31, 2017, the net transfer out of Level 3 was exclusively to Level 2.