Summary of Significant Accounting Policies - Commitments and Contingencies (Details)	3 Months Ended
	Mar. 31, 2019 USD ($) ft²	Mar. 31, 2018 USD ($)
Accounting Policies [Abstract]
Operating cash flows for lease liabilities	$ 14,080,000
Square feet of office space | ft²	60,293
Monthly rent payments	$ 97,976
Rent expense	300,000
Lease, Cost [Abstract]
Operating lease cost	15,070,000	$ 14,553,000
Variable lease cost	$ 3,259,000	$ 3,021,000
Weighted-average remaining lease term (years)	5 years 2 months 16 days
Weighted-average discount rate	3.50%
Operating Lease Liabilities, Payments Due [Abstract]
2019	$ 38,389,000
2020	50,443,000
2021	39,184,000
2022	29,883,000
2023	18,089,000
Thereafter	17,554,000
Total lease payments	193,542,000
Less: interest	15,028,000
Present value of lease liabilities	$ 178,514,000