Investments - Amortization by Expected Maturity (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Amortized cost of fixed maturities available-for-sale
Due in one year or less	$ 2,170.7
Due after one year through five years	10,179.8
Due after five years through ten years	12,378.6
Due after ten years	22,694.6
Subtotal	47,423.7
Mortgage-backed and other asset-backed securities	19,225.0
Amortized cost, fixed maturities	66,648.7	$ 65,302.5
Fair value of fixed maturities available-for-sale
Due in one year or less	2,173.4
Due after one year through five years	10,168.8
Due after five years through ten years	12,459.7
Due after ten years	25,070.6
Subtotal	49,872.5
Mortgage-backed and other asset-backed securities	18,835.3
Fair value, fixed maturities	$ 68,707.8	$ 70,106.2