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Investment Securities (Net Realized and Unrealized Investement Gains and Losses) (Details) - USD ($)
 3 Months Ended 12 Months Ended
Mar. 31, 2019
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]        
Net realized investment gains (losses), pre-tax (1)   $ 96,600,000 $ (12,900,000) $ 24,100,000
Net unrealized investment gains (losses), pre-tax (2)   336,600,000 (95,400,000) 109,200,000
Net realized and unrealized investment gains (losses), pre-tax   433,200,000 (108,300,000) 133,300,000
Income tax (expense) benefit attributable to net realized and unrealized investment gains (losses)   (87,000,000.0) 18,200,000 (12,900,000)
Net realized and unrealized investment gains (losses), after-tax   346,200,000 $ (90,100,000) $ 120,400,000
MediaAlpha        
Debt Securities, Available-for-sale [Line Items]        
Realized gain from transaction $ 67,500,000 67,500,000    
Unrealized investment gain $ 114,700,000 $ 180,000,000.0