Nature of Operations and Significant Accounting Policies - Derivatives (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Derivatives
Other investments	$ 3,586.2	$ 3,655.9
Other liabilities	$ 5,294.1	5,783.3
Interest rate contracts | Over-The-Counter Derivatives Cleared on Chicago Mercantile Exchange
Derivatives
Other investments		528.0
Other liabilities		$ 527.7