Significant Accounting Policies - New Accounting Pronouncements (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Jan. 01, 2019	Dec. 31, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease assets	$ 937,884
Present value of lease liabilities	958,066
Direct costs of leased and rented property or equipment	$ 13,000
Cumulative effect on retained earnings, net of tax			$ 28,944	$ 5,074
Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect on retained earnings, net of tax			$ 28,944	$ 5,074
Retained Earnings | ASU 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect on retained earnings, net of tax		$ 28,944