Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Financial Assets:
Fair Value	$ 2,976,089	$ 2,760,066
U.S. Treasury securities
Financial Assets:
Fair Value	286,843	289,892
U.S. agency securities
Financial Assets:
Fair Value	33,187	32,997
U.S. agency mortgage-backed securities
Financial Assets:
Fair Value	672,142	637,178
Municipal debt securities
Financial Assets:
Fair Value	461,382	483,879
Non-U.S. government securities:
Financial Assets:
Fair Value	46,366	45,001
Corporate debt securities
Financial Assets:
Fair Value	763,401	725,201
Residential and commercial mortgage securities
Financial Assets:
Fair Value	187,210	121,838
Asset-backed securities
Financial Assets:
Fair Value	314,736	284,997
Money market funds
Financial Assets:
Fair Value	210,822	139,083
Fair Value, Measurements, Recurring
Financial Assets:
Total assets at fair value	2,976,089	2,760,066
Fair Value, Measurements, Recurring | U.S. Treasury securities
Financial Assets:
Fair Value	286,843	289,892
Fair Value, Measurements, Recurring | U.S. agency securities
Financial Assets:
Fair Value	33,187	32,997
Fair Value, Measurements, Recurring | U.S. agency mortgage-backed securities
Financial Assets:
Fair Value	672,142	637,178
Fair Value, Measurements, Recurring | Municipal debt securities
Financial Assets:
Fair Value	461,382	483,879
Fair Value, Measurements, Recurring | Non-U.S. government securities:
Financial Assets:
Fair Value	46,366	45,001
Fair Value, Measurements, Recurring | Corporate debt securities
Financial Assets:
Fair Value	763,401	725,201
Fair Value, Measurements, Recurring | Residential and commercial mortgage securities
Financial Assets:
Fair Value	187,210	121,838
Fair Value, Measurements, Recurring | Asset-backed securities
Financial Assets:
Fair Value	314,736	284,997
Fair Value, Measurements, Recurring | Money market funds
Financial Assets:
Fair Value	210,822	139,083
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Instruments (Level 1)
Financial Assets:
Total assets at fair value	497,665	428,975
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Instruments (Level 1) | U.S. Treasury securities
Financial Assets:
Fair Value	286,843	289,892
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	210,822	139,083
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2)
Financial Assets:
Total assets at fair value	2,478,424	2,331,091
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	33,187	32,997
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | U.S. agency mortgage-backed securities
Financial Assets:
Fair Value	672,142	637,178
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Municipal debt securities
Financial Assets:
Fair Value	461,382	483,879
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Non-U.S. government securities:
Financial Assets:
Fair Value	46,366	45,001
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Corporate debt securities
Financial Assets:
Fair Value	763,401	725,201
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Residential and commercial mortgage securities
Financial Assets:
Fair Value	187,210	121,838
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Asset-backed securities
Financial Assets:
Fair Value	314,736	284,997
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