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Nature of Operations and Significant Accounting Policies - Recent Accounting Pronouncements (Details) - USD ($)
$ in Millions
6 Months Ended
Jun. 30, 2018
Dec. 31, 2017
Recent Accounting Pronouncements    
Equity securities, available-for-sale   $ 96.0
Equity securities, trading   $ 1,770.6
ASU 2016-16 - Intra-entity asset transfer taxes    
Recent Accounting Pronouncements    
Cumulative effect adjustment, increase to retained earnings $ 8.7  
Cumulative effect adjustment, decrease to other assets 21.1  
Cumulative effect adjustment, decrease to deferred income taxes 29.8  
Accumulated other comprehensive income (loss) | ASU 2018-02 - Reclassification of certain tax effects    
Recent Accounting Pronouncements    
Cumulative effect adjustment from adopting guidance 77.6  
Accumulated other comprehensive income (loss) | ASU 2016-01 - Equity investments    
Recent Accounting Pronouncements    
Cumulative effect adjustment from adopting guidance (1.0)  
Retained earnings (accumulated deficit) | ASU 2018-02 - Reclassification of certain tax effects    
Recent Accounting Pronouncements    
Cumulative effect adjustment from adopting guidance (77.6)  
Retained earnings (accumulated deficit) | ASU 2016-16 - Intra-entity asset transfer taxes    
Recent Accounting Pronouncements    
Cumulative effect adjustment, increase to retained earnings 8.7  
Retained earnings (accumulated deficit) | ASU 2016-01 - Equity investments    
Recent Accounting Pronouncements    
Cumulative effect adjustment from adopting guidance $ 1.0