Nature of Operations and Significant Accounting Policies - Recent Accounting Pronouncements (Details) - USD ($) $ in Millions	Jan. 01, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Recent Accounting Pronouncements
Total stockholders' equity		$ 10,001.7	$ 16,069.4
AOCI
Recent Accounting Pronouncements
Total stockholders' equity		$ (7,199.0)	$ 1,610.9	$ 2,383.1	$ 1,037.9
ASU 2018-12 - Targeted improvements to the accounting for long-duration insurance contracts | Effects of implementation of accounting change
Recent Accounting Pronouncements
Total stockholders' equity	$ (5,300.0)
ASU 2018-12 - Targeted improvements to the accounting for long-duration insurance contracts | Retained earnings (accumulated deficit) | Effects of implementation of accounting change
Recent Accounting Pronouncements
Total stockholders' equity	(200.0)
ASU 2018-12 - Targeted improvements to the accounting for long-duration insurance contracts | AOCI | Effects of implementation of accounting change
Recent Accounting Pronouncements
Total stockholders' equity	$ (5,100.0)