Investments - Amortization by Expected Maturity (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Amortized cost of fixed maturities available-for-sale
Due in one year or less	$ 2,138.3
Due after one year through five years	9,955.3
Due after five years through ten years	12,080.0
Due after ten years	22,155.9
Subtotal	46,329.5
Mortgage-backed and other asset-backed securities	18,973.0
Amortized cost, fixed maturities	65,302.5	$ 59,725.8
Fair value of fixed maturities available-for-sale
Due in one year or less	2,154.0
Due after one year through five years	10,319.2
Due after five years through ten years	12,894.4
Due after ten years	25,490.2
Subtotal	50,857.8
Mortgage-backed and other asset-backed securities	19,248.4
Fair value, fixed maturities	$ 70,106.2	$ 60,108.5