Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents, at carrying amount	$ 100,094	$ 42,052	$ 87,977	$ 71,491
Debt, at carrying amount	989,380	905,283
Notes payable – related party, at carrying amount	285,000	370,000
Senior Notes [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt, at carrying amount	989,380	495,283
Subordinated Debt [Member] | Majority Shareholder [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable – related party, at carrying amount	285,000	370,000
Revolver [Member] | Line of Credit [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt, at carrying amount	0	410,000
Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents, at fair value	100,094	42,052
Level 2 [Member] | Senior Notes [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt, at fair value	986,930	523,800
Level 2 [Member] | Subordinated Debt [Member] | Majority Shareholder [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes payable – related party, at fair value	285,000	370,000
Level 2 [Member] | Revolver [Member] | Line of Credit [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt, at fair value	$ 0	$ 410,000