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Purchase price of common stock expressed as a percentage of its fair value.

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Stock-Based Compensation (Details) - $ / shares	3 Months Ended		12 Months Ended
Stock-Based Compensation (Details) - $ / shares	Mar. 31, 2019	Apr. 01, 2018	Dec. 31, 2018
Second Amended and Restated 2004 Stock Incentive Plan
Stock-based compensation
Vesting period	3 years
Second Amended and Restated 2004 Stock Incentive Plan \| Restricted stock
Stock-based compensation
Granted (in shares)	89,053
Second Amended and Restated 2004 Stock Incentive Plan \| Deferred shares
Stock-based compensation
Granted (in shares)		105,902
Second Amended and Restated 2004 Stock Incentive Plan \| Performance stock units
Stock-based compensation
Vesting period	3 years
Granted (in shares)	82,898	94,215
Management Stock Purchase Plan \| Maximum
Stock-based compensation
Percentage of annual incentive bonus that may be used to purchase RSU's	50.00%
Management Stock Purchase Plan \| Class A
Stock-based compensation
Purchase price as percentage of fair market value of common stock on grant date	80.00%
Management Stock Purchase Plan \| Restricted stock units (RSUs)
Stock-based compensation
Granted (in shares)	36,670	36,208
Fair value assumptions
Expected life (years)	3 years		3 years
Expected stock price volatility (as a percent)	23.30%		24.10%
Expected dividend yield (as a percent)	1.10%		1.00%
Risk-free interest rate (as a percent)	2.50%		2.40%
Weighted average grant-date fair value (in dollars per share)	$ 22.16	$ 21.80