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Debt - Senior Debentures (Details) - 6.70% Senior Debentures due 2034 - USD ($)
$ in Millions
Dec. 31, 2019
Dec. 31, 2018
Oct. 30, 2015
Sep. 09, 2015
Debt Instrument [Line Items]        
Interest rate, stated percentage 6.70% 6.70%   6.70%
Debt acquired in an acquisition       $ 300
Annual effective interest rate     10.96%