Investments Distribution by External Credit Rating for Fixed Maturity Securities (Details) $ in Millions	Mar. 31, 2018 USD ($)
Schedule of Available-for-sale Securities
Fixed Maturity Securities	$ 44,500.1
Accumulated Gross Unrealized Gain on Securities	4,641.3
Accumulated Gross Unrealized Loss on Securities	$ 293.1
Percent of Fixed Maturity Securities in Unrealized Loss Position	100.00%
External Credit Rating, Investment Grade
Schedule of Available-for-sale Securities
Fixed Maturity Securities	$ 41,233.8
Accumulated Gross Unrealized Gain on Securities	4,527.8
Accumulated Gross Unrealized Loss on Securities	$ 198.7
Percent of Fixed Maturity Securities in Unrealized Loss Position	67.80%
External Credit Rating, Non Investment Grade
Schedule of Available-for-sale Securities
Fixed Maturity Securities	$ 3,266.3
Accumulated Gross Unrealized Gain on Securities	113.5
Accumulated Gross Unrealized Loss on Securities	$ 94.4
Percent of Fixed Maturity Securities in Unrealized Loss Position	32.20%