Long-Term Debt (Details) - USD ($)		6 Months Ended	12 Months Ended
	Jul. 31, 2018	Jun. 30, 2018	Dec. 31, 2017	Dec. 31, 2016	Mar. 09, 2018	Jan. 31, 2018	Jan. 30, 2018
Line of Credit Facility [Line Items]
Unamortized debt issuance expense		$ 3,100,000	$ 1,400,000
Revolving Credit Facility
Line of Credit Facility [Line Items]
Maximum borrowing capacity					$ 800,000,000	$ 530,000,000	$ 350,000,000
Additional borrowing capacity available		350,000,000
Outstanding		$ 530,000,000	$ 85,000,000
Rate during period		3.25%	2.75%
Commitment fee		0.20%		0.20%
Revolving Credit Facility | Overnight Bank Funding Rate
Line of Credit Facility [Line Items]
Basis spread		0.50%
Revolving Credit Facility | Federal Funds Effective Swap Rate
Line of Credit Facility [Line Items]
Basis spread		0.50%
Revolving Credit Facility | London Interbank Offered Rate (LIBOR)
Line of Credit Facility [Line Items]
Basis spread		1.00%
Subsequent Event | Revolving Credit Facility
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 500,000,000
Minimum | Subsequent Event | Revolving Credit Facility | Base Rate
Line of Credit Facility [Line Items]
Basis spread	0.00%
Minimum | Subsequent Event | Revolving Credit Facility | Eurodollar
Line of Credit Facility [Line Items]
Basis spread	1.00%
Maximum | Subsequent Event | Revolving Credit Facility | Base Rate
Line of Credit Facility [Line Items]
Basis spread	0.50%
Maximum | Subsequent Event | Revolving Credit Facility | Eurodollar
Line of Credit Facility [Line Items]
Basis spread	1.50%