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Employee Compensation Plans - Summarizes of Company's Non-Vested ESP/EIP Awards (Details) - Common Class A [Member] - Restricted Stock - $ / shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]    
Number of shares, Non-vested beginning of year 1,223,533  
Granted, Number of shares 464,100  
Vested, Number of shares (548,062)  
Forfeited or expired, Number of shares (72,275)  
Number of shares, Non-vested end of year 1,067,296 1,223,533
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Additional Disclosures    
Weighted average fair value, Non-vested beginning of year $ 17.11  
Weighted average fair value, Granted 17.01  
Weighted average fair value, Vested 18.59  
Weighted average fair value, Forfeited or expired 16.50  
Weighted average fair value, Non-vested end of year $ 16.34 $ 17.11
Remaining contractual life, Non-vested 2 years 2 months 24 days 2 years 19 days
Remaining contractual life, Granted 1 year 11 months 4 days