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Stock-Based Compensation Plans (Tables)
 9 Months Ended
Sep. 30, 2021
Share-based Payment Arrangement [Abstract]  
Summary of Stock Option Activity A summary of stock option activity is as follows:
Number of
Shares		Weighted Average
Exercise Price ($)		Weighted Average
Remaining Contractual
Term (Years)		Aggregate Intrinsic Value
of In-the-Money
Options ($)
Outstanding as of December 31, 20202,186,511 $17.87 
Exercised(380,676)19.05 
Outstanding as of September 30, 20211,805,835 $17.63 3.27$23,662,370 
Exercisable as of September 30, 20211,805,835 $17.63 3.27$23,662,370 
Summary of Nonvested Performance Award Activity A summary of nonvested total shareholder return awards ("TSRs") is as follows:
Number of
Shares		Weighted Average
Grant Date Fair Value
Nonvested as of December 31, 20201,367,728 $34.59 
Granted367,317 50.60 
Vested(782,588)31.31 
Forfeited(153,681)38.57 
Change in payout rate391,294 31.31 
Nonvested as of September 30, 20211,190,070 $40.10 
Summary of Grant Date Fair Value Weighted-Average Assumptions, Awards Other Than Options
The fair value of TSRs granted during the nine months ended September 30, 2021 and 2020, were estimated on the date of grant using the Monte Carlo simulation model, acceptable under ASC 718, Compensation - Stock Compensation, using the following weighted average assumptions:

Nine Months Ended September 30,
20212020
Expected life (years)2.82.8
Risk-free interest rate0.3 %0.5 %
Expected volatility41.2 %31.4 %
Expected dividend yield— — 
Summary of Nonvested Restricted Share Unit Activity A summary of nonvested restricted share unit awards ("RSUs") is as follows:
Number of
Shares		Weighted Average
Grant Date Fair Value
Nonvested as of December 31, 20201,118,182 $27.34 
Granted578,220 39.27 
Vested(522,608)27.69 
Forfeited(232,734)30.54 
Nonvested as of September 30, 2021941,060 $33.69