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Share-based Compensation (Details) - USD ($)
3 Months Ended 12 Months Ended
Apr. 21, 2015
Apr. 15, 2015
Jul. 08, 2011
Mar. 31, 2018
Mar. 31, 2017
Dec. 31, 2017
Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Non-voting common interest units outstanding (in shares)       11,512,297   12,301,067  
Number of non-voting common interest units forfeited or repurchased (in shares)       788,770   729,272  
Class A              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Accrued compensation expense       $ 3,900,000 $ 4,700,000    
Granted (in shares)       594,536      
Granted during period, value       $ 11,300,000      
Class B interests | East MIP              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Vesting period     4 years        
Non-voting common interest units (formerly Class B interests)              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Expense recognized       0 200,000    
Unrecognized share-based compensation expense       $ 0   $ 100,000  
Weighted average period for compensation expense expected to be recognized       0 years   9 months 18 days  
Capitalized software development costs       $ 0 10,000    
Non-voting common interest units (formerly Class B interests) | East MIP              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Amortization expense related to share based compensation       10,000 0    
Non-voting common interest units (formerly Class A 2 profits interests)              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Expense recognized       0 200,000    
Unrecognized share-based compensation expense       $ 0   $ 100,000  
Weighted average period for compensation expense expected to be recognized       0 years   1 month 6 days  
Non-voting common interest units (formerly Class A 2 profits interests) | Maximum              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Vesting period   4 years          
Non-qualified stock options | 2015 Management Incentive Plan              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Vesting period 4 years            
Expiration period 10 years            
Expense recognized       $ 1,400,000 $ 1,400,000    
Unrecognized share-based compensation expense       $ 6,100,000   $ 7,500,000  
Weighted average period for compensation expense expected to be recognized       1 year 1 month 6 days   1 year 3 months 18 days  
Forfeited or repurchased (in shares)         (195,000)    
Outstanding (in shares)       7,783,000 8,234,000 8,234,000  
Forfeited or expired (in shares)         (195,000)    
Outstanding (in shares)       7,051,000 8,039,000 7,783,000 8,234,000
Weighted Average Exercise Price Per Share              
Outstanding (in dollars per share)       $ 19.00 $ 19.00 $ 19.00  
Exercised (in dollars per share)       19.00      
Outstanding (in dollars per share)       $ 19.00 $ 19.00 $ 19.00 $ 19.00
Activity              
Exercised (in shares)       (732,000)      
Weighted Average Remaining Life, outstanding              
Weighted Average Remaining Life       7 years 18 days 8 years 18 days 7 years 3 months 15 days 8 years 3 months 15 days
Options Exercisable, Number of Options (in shares)       3,182,000 2,009,750 3,891,500 2,058,500
Options Exercisable, Weighted Average Exercise Price (in dollars per share)       $ 19.00 $ 19.00 $ 19.00 $ 19.00
Restricted stock units              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Unrecognized share-based compensation expense       $ 34,800,000   $ 14,300,000  
Weighted average period for compensation expense expected to be recognized       1 year 10 months 24 days   1 year 6 months  
Restricted stock units | Class A              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Expense recognized       $ 4,000,000 $ 2,600,000    
Restricted stock units | 2015 Management Incentive Plan              
Activity              
Outstanding (in shares)       853,047 1,573,441 1,573,441  
Granted (in shares)       1,044,690 0    
Forfeited (in shares)       (30,626) (95,481)    
Vested (in shares)       0 0    
Outstanding (in shares)       1,867,111 1,477,960 853,047 1,573,441
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract]              
Outstanding (in dollars per share)       $ 17.94 $ 18.28 $ 18.28  
Granted (in dollars per share)       20.64 0.00    
Forfeited (in dollars per share)       18.52 18.38    
Vested (in dollars per share)       0.00 0.00    
Outstanding (in dollars per share)       $ 18.85 $ 18.28 $ 17.94 $ 18.28
Restricted stock units | 2015 Management Incentive Plan | Maximum              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Vesting period       4 years