Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Financial Assets:
Fair Value	$ 4,502,349	$ 3,350,747
Fair Value, Measurements, Recurring
Financial Assets:
Total assets at fair value	4,502,349	3,350,747
Fair Value, Measurements, Recurring | U.S. Treasury securities
Financial Assets:
Fair Value	229,417	242,206
Fair Value, Measurements, Recurring | U.S. agency securities
Financial Assets:
Fair Value	14,640	33,605
Fair Value, Measurements, Recurring | U.S. agency mortgage-backed securities
Financial Assets:
Fair Value	956,581	848,334
Fair Value, Measurements, Recurring | Municipal debt securities
Financial Assets:
Fair Value	493,673	361,638
Fair Value, Measurements, Recurring | Non-U.S. government securities:
Financial Assets:
Fair Value	60,752	54,995
Fair Value, Measurements, Recurring | Corporate debt securities
Financial Assets:
Fair Value	985,384	880,301
Fair Value, Measurements, Recurring | Residential and commercial mortgage securities
Financial Assets:
Fair Value	366,227	288,281
Fair Value, Measurements, Recurring | Asset-backed securities
Financial Assets:
Fair Value	442,723	326,025
Fair Value, Measurements, Recurring | Money market funds
Financial Assets:
Fair Value	952,952	315,362
Fair Value, Measurements, Recurring | Quoted Prices in Active  Markets for Identical Instruments (Level 1)
Financial Assets:
Total assets at fair value	1,182,369	557,568
Fair Value, Measurements, Recurring | Quoted Prices in Active  Markets for Identical Instruments (Level 1) | U.S. Treasury securities
Financial Assets:
Fair Value	229,417	242,206
Fair Value, Measurements, Recurring | Quoted Prices in Active  Markets for Identical Instruments (Level 1) | U.S. agency securities
Financial Assets:
Fair Value	0	0
Fair Value, Measurements, Recurring | Quoted Prices in Active  Markets for Identical Instruments (Level 1) | U.S. agency mortgage-backed securities
Financial Assets:
Fair Value	0	0
Fair Value, Measurements, Recurring | Quoted Prices in Active  Markets for Identical Instruments (Level 1) | Municipal debt securities
Financial Assets:
Fair Value	0	0
Fair Value, Measurements, Recurring | Quoted Prices in Active  Markets for Identical Instruments (Level 1) | Non-U.S. government securities:
Financial Assets:
Fair Value	0	0
Fair Value, Measurements, Recurring | Quoted Prices in Active  Markets for Identical Instruments (Level 1) | Corporate debt securities
Financial Assets:
Fair Value	0	0
Fair Value, Measurements, Recurring | Quoted Prices in Active  Markets for Identical Instruments (Level 1) | Residential and commercial mortgage securities
Financial Assets:
Fair Value	0	0
Fair Value, Measurements, Recurring | Quoted Prices in Active  Markets for Identical Instruments (Level 1) | Asset-backed securities
Financial Assets:
Fair Value	0	0
Fair Value, Measurements, Recurring | Quoted Prices in Active  Markets for Identical Instruments (Level 1) | Money market funds
Financial Assets:
Fair Value	952,952	315,362
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2)
Financial Assets:
Total assets at fair value	3,319,980	2,793,179
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | U.S. Treasury securities
Financial Assets:
Fair Value	0	0
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | U.S. agency securities
Financial Assets:
Fair Value	14,640	33,605
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | U.S. agency mortgage-backed securities
Financial Assets:
Fair Value	956,581	848,334
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Municipal debt securities
Financial Assets:
Fair Value	493,673	361,638
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Non-U.S. government securities:
Financial Assets:
Fair Value	60,752	54,995
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Corporate debt securities
Financial Assets:
Fair Value	985,384	880,301
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Residential and commercial mortgage securities
Financial Assets:
Fair Value	366,227	288,281
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Asset-backed securities
Financial Assets:
Fair Value	442,723	326,025
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Money market funds
Financial Assets:
Fair Value	0	0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3)
Financial Assets:
Total assets at fair value	0	0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | U.S. Treasury securities
Financial Assets:
Fair Value	0	0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | U.S. agency securities
Financial Assets:
Fair Value	0	0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | U.S. agency mortgage-backed securities
Financial Assets:
Fair Value	0	0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Municipal debt securities
Financial Assets:
Fair Value	0	0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Non-U.S. government securities:
Financial Assets:
Fair Value	0	0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Corporate debt securities
Financial Assets:
Fair Value	0	0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Residential and commercial mortgage securities
Financial Assets:
Fair Value	0	0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Asset-backed securities
Financial Assets:
Fair Value	0	0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Money market funds
Financial Assets:
Fair Value	$ 0	$ 0