Funds Withheld Receivables Available-for-sale Securities, Continuous Unrealized Loss (Details) - USD ($) $ in Millions	Sep. 30, 2016	Sep. 30, 2015
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	$ 406.7	$ 765.6
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	38.3	76.1
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	216.0	549.7
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	15.7	52.0
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	190.7	215.9
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	22.6	24.1
Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	229.5	531.2
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	29.5	67.9
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	137.8	417.4
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	12.6	46.9
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	91.7	113.8
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	16.9	21.0
Mortgage Backed Securities, Other [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	172.3	209.5
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	7.9	6.6
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	73.3	120.8
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	2.2	3.8
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	99.0	88.7
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	5.7	2.8
Municipal Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	1.0	19.3
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	0.0	0.7
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	1.0	5.9
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0.0	0.4
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0.0	13.4
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0.0	0.3
United States Government and government agencies and authorities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	0.2
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	0.0
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	0.2
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0.0
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0.0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0.0
Preferred Stock [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	3.7	5.6
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	0.9	0.9
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	3.7	5.6
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0.9	0.9
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0.0	0.0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	$ 0.0	$ 0.0