Lease Agreements - Company as Lessee (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Lessee, Lease, Description [Line Items]
Office lease, renewal term	5 years
Operating lease right-of-use assets	$ 76,996	$ 0
Total leased assets	76,996
Operating lease liabilities	79,010
Total lease liabilities	79,010
Lease, Cost [Abstract]
Operating lease cost	1,686
Variable lease cost	142
Short-term lease cost	132
Sublease income	(107)
Total lease cost	$ 1,853
Minimum
Lessee, Lease, Description [Line Items]
Office lease term of contract	5 years
Ground lease term	20 years
Ground lease, renewal term	10 years
Maximum
Lessee, Lease, Description [Line Items]
Office lease term of contract	10 years
Ground lease term	85 years
Ground lease, renewal term	39 years
Operating lease right-of-use assets
Lessee, Lease, Description [Line Items]
Operating lease right-of-use assets	$ 76,996
Operating lease liabilities
Lessee, Lease, Description [Line Items]
Operating lease liabilities	$ 79,010