Shareholders' Equity / Partners' Capital - Stock Options Activity (Details) - 2012 Equity Incentive Plan - Stock options - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
Stock Options
Outstanding at beginning of the period (in shares)	2,252,275	3,052,450	3,052,450
Granted (in shares)	20,000	140,000
Exercised (in shares)	(5,000)	(7,500)
Forfeited (in shares)	(9,850)	(10,000)
Outstanding at end of the period (in shares)	2,257,425	3,174,950	2,252,275	3,052,450
Exercisable at end of the period (in shares)	1,867,025	2,272,675
Weighted Average Exercise Price
Outstanding at beginning of the period (in dollars per share)	$ 16.92	$ 16.65	$ 16.65
Granted (in dollars per share)	20.48	19.40
Exercised (in dollars per share)	14.00	16.62
Forfeited (in dollars per share)	22.53	17.83
Outstanding at end of the period (in dollars per share)	16.93	16.76	$ 16.92	$ 16.65
Exercisable at end of the period (in dollars per share)	$ 16.41	$ 16.16
Aggregate intrinsic value
Weighted average remaining life (in years)	5 years 10 months 21 days	6 years 8 months 12 days	6 years 1 month 6 days	6 years 10 months 24 days
Weighted average exercisable (in years)	5 years 5 months 16 days	6 years 1 month 6 days
Aggregate intrinsic value	$ 13,267	$ 11,748	$ 7,713	$ 16,421
Aggregate intrinsic value exercised	35	22
Aggregate intrinsic value exercisable	$ 11,872	$ 9,213

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The number of shares into which fully or partially vested stock options outstanding as of the balance sheet date can be currently converted under the option plan.

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The weighted-average price as of the balance sheet date at which grantees can acquire the shares reserved for issuance on vested portions of options outstanding and currently exercisable under the stock option plan.

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Amount of accumulated difference between fair value of underlying shares on dates of exercise and exercise price on options exercised (or share units converted) into shares.

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The number of shares under options that were cancelled during the reporting period as a result of occurrence of a terminating event specified in contractual agreements pertaining to the stock option plan.

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Net number of share options (or share units) granted during the period.

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Amount by which the current fair value of the underlying stock exceeds the exercise price of options outstanding.

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Number of options outstanding, including both vested and non-vested options.

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan.

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Weighted average price at which option holders acquired shares when converting their stock options into shares.

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Weighted average price at which grantees could have acquired the underlying shares with respect to stock options that were terminated.

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Weighted average per share amount at which grantees can acquire shares of common stock by exercise of options.

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Amount of difference between fair value of the underlying shares reserved for issuance and exercise price of vested portions of options outstanding and currently exercisable.

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Weighted average remaining contractual term for vested portions of options outstanding and currently exercisable or convertible, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Weighted average remaining contractual term for option awards outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Number of share options (or share units) exercised during the current period.

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