Employee Benefit Plans (Tables)

12 Months Ended

Dec. 31, 2017

Employee benefit plans [Line Items]

Restricted Stock Award Activity

Restricted Stock Award Activity

Schedule of Weighted Average Assumptions Used

The Company uses a Monte Carlo model to value its performance restricted stock units.

Monte Carlo Pricing

Weighted Average Assumptions

Stock Purchase Plan

Stock Purchase Plan

Share-Based Compensation Expense Summary

Share‑Based Compensation Expense Summary

Stock Options [Member]

Employee benefit plans [Line Items]

Schedule of Stock Options

Time Vested Stock Options

	Options for Common Shares		Weighted Average Exercise Price		Number of Exercisable Options
Balance as of December 31, 2016	1,170,593		$	18.43	1,145,356
Options granted	—		—
Options exercised	(331,639	)	21.02
Options forfeited/expired	—		—
Balance as of December 31, 2017	838,954		$	17.41	838,954

Performance Stock Options [Member]

Employee benefit plans [Line Items]

Schedule of Stock Options

Performance Stock Options

Restricted Stock Units (RSUs) [Member]

Employee benefit plans [Line Items]

Restricted Stock Unit Activity (Excluding Dividend Equivalents)

Restricted Stock Unit Activity

Performance Restricted Stock Units [Member]

Employee benefit plans [Line Items]

Restricted Stock Unit Activity (Excluding Dividend Equivalents)

Performance Restricted Stock Unit Activity

Performance Restricted Stock Units	Number of Performance Share Units		Weighted Average Grant Date Fair Value Per Share
Nonvested at December 31, 2016	609,435		$	26.22
Granted (1)	315,896		53.74
Delivered	(318,467	)	25.17
Forfeited	—		—
Nonvested at December 31, 2017 (2)	606,864		$	33.80

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(1) Includes 155,000 performance restricted stock units that were granted prior to 2017 at a weighted average grant date fair value of $25.17, but met performance hurdles and vested in February 2017.  The weighted average grant date fair value per share excludes these shares.

(2) Excludes 426,670 performance restricted stock units that have met performance hurdles and will be eligible for vesting after December 31, 2017