Leases - Ground Leases (Details) ft² in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2019 USD ($) ft² property Lease	Mar. 31, 2018 USD ($)	Dec. 31, 2018 ft² property Lease	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Lessee, Lease, Description [Line Items]
Number of properties subject to ground leases | property	109		107
Square feet subject to ground leases | ft²	9.1		8.8
Number of prepaid ground leases	46
Amortization of prepaid rent | $	$ 100,000	$ 100,000		$ 500,000	$ 500
Number of non-prepaid ground leases	63		60
Weighted Average Remaining Term
Lessee, Lease, Description [Line Items]
Ground lease, initial term			53 years 10 months 24 days
Minimum
Lessee, Lease, Description [Line Items]
Ground lease, initial term			50 years
Term of lease, including renewals			75 years
Maximum
Lessee, Lease, Description [Line Items]
Ground lease, initial term			75 years
Term of lease, including renewals			100 years
Accounting Standards Update 2016-02 | Weighted Average Remaining Term
Lessee, Lease, Description [Line Items]
Ground lease, initial term	54 years 10 months 24 days
Term of lease, including renewals	70 years 8 months 12 days
Accounting Standards Update 2016-02 | Minimum
Lessee, Lease, Description [Line Items]
Ground lease, initial term	40 years
Accounting Standards Update 2016-02 | Maximum
Lessee, Lease, Description [Line Items]
Ground lease, initial term	99 years