Earnings Per Share - Calculation of Basic and Diluted Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Earnings Per Share [Abstract]
Net income	$ 97,446	$ 86,700	$ 176,611	$ 152,414
Income attributable to unvested participating securities, basic EPS	(428)	(483)	(835)	(917)
Net income used in calculating basic EPS	$ 97,018	$ 86,217	$ 175,776	$ 151,497
Weighted-average vested shares	42,483	44,066	42,653	44,401
Basic EPS	$ 2.28	$ 1.96	$ 4.12	$ 3.41
Income attributable to unvested participating securities, diluted EPS	$ (427)	$ (482)	$ (833)	$ (915)
Net income used in calculating diluted EPS	$ 97,019	$ 86,218	$ 175,778	$ 151,499
Dilutive effect of incremental shares to be issued for contingently-issuable shares	136	141	127	128
Weighted-average shares used in calculating diluted EPS	42,619	44,207	42,780	44,529
Diluted EPS	$ 2.28	$ 1.95	$ 4.11	$ 3.40

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The amount of net income (loss) for the period per each share of common stock or unit outstanding during the reporting period.

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The amount of net income (loss) for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

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Additional shares included in the calculation of diluted EPS as a result of the potentially dilutive effect of share based payment arrangements using the treasury stock method.

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The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

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