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Stock-Based Compensation Plans - Summary of Nonvested LTIP Performance Shares (Details) - LTIP Performance Shares
6 Months Ended
Jun. 30, 2019
$ / shares
shares
Number of Shares at Expected Attainment  
Nonvested, beginning balance (in shares) | shares 540,697
Forfeited (in shares) | shares (16,319)
Change in attainment (in shares) | shares 377,557
Nonvested, ending balance (in shares) | shares 901,935
Weighted Average Grant Date Fair Value  
Nonvested, beginning balance (in dollars per share) | $ / shares $ 19.83
Forfeited (in dollars per share) | $ / shares 20.12
Change in attainment (in dollars per share) | $ / shares 20.22
Nonvested, ending balance (in dollars per share) | $ / shares $ 19.99