Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Recurring fair value measurements
Total investments available for sale	$ 4,565,373	$ 3,350,747
Fair Value, Recurring
Recurring fair value measurements
Total assets at fair value	4,565,373	3,350,747
Fair Value, Recurring | U.S. Treasury securities
Recurring fair value measurements
Total investments available for sale	268,444	242,206
Fair Value, Recurring | U.S. agency securities
Recurring fair value measurements
Total investments available for sale	18,085	33,605
Fair Value, Recurring | U.S. agency mortgage-backed securities
Recurring fair value measurements
Total investments available for sale	995,905	848,334
Fair Value, Recurring | Municipal debt securities
Recurring fair value measurements
Total investments available for sale	551,517	361,638
Fair Value, Recurring | Non-U.S. government securities
Recurring fair value measurements
Total investments available for sale	61,607	54,995
Fair Value, Recurring | Corporate debt securities
Recurring fair value measurements
Total investments available for sale	1,126,512	880,301
Fair Value, Recurring | Residential and commercial mortgage securities
Recurring fair value measurements
Total investments available for sale	409,282	288,281
Fair Value, Recurring | Asset-backed securities
Recurring fair value measurements
Total investments available for sale	454,717	326,025
Fair Value, Recurring | Money market funds
Recurring fair value measurements
Total investments available for sale	679,304	315,362
Fair Value, Recurring | Quoted Prices in Active  Markets for Identical Instruments (Level 1)
Recurring fair value measurements
Total assets at fair value	947,748	557,568
Fair Value, Recurring | Quoted Prices in Active  Markets for Identical Instruments (Level 1) | U.S. Treasury securities
Recurring fair value measurements
Total investments available for sale	268,444	242,206
Fair Value, Recurring | Quoted Prices in Active  Markets for Identical Instruments (Level 1) | U.S. agency securities
Recurring fair value measurements
Total investments available for sale	0	0
Fair Value, Recurring | Quoted Prices in Active  Markets for Identical Instruments (Level 1) | U.S. agency mortgage-backed securities
Recurring fair value measurements
Total investments available for sale	0	0
Fair Value, Recurring | Quoted Prices in Active  Markets for Identical Instruments (Level 1) | Municipal debt securities
Recurring fair value measurements
Total investments available for sale	0	0
Fair Value, Recurring | Quoted Prices in Active  Markets for Identical Instruments (Level 1) | Non-U.S. government securities
Recurring fair value measurements
Total investments available for sale	0	0
Fair Value, Recurring | Quoted Prices in Active  Markets for Identical Instruments (Level 1) | Corporate debt securities
Recurring fair value measurements
Total investments available for sale	0	0
Fair Value, Recurring | Quoted Prices in Active  Markets for Identical Instruments (Level 1) | Residential and commercial mortgage securities
Recurring fair value measurements
Total investments available for sale	0	0
Fair Value, Recurring | Quoted Prices in Active  Markets for Identical Instruments (Level 1) | Asset-backed securities
Recurring fair value measurements
Total investments available for sale	0	0
Fair Value, Recurring | Quoted Prices in Active  Markets for Identical Instruments (Level 1) | Money market funds
Recurring fair value measurements
Total investments available for sale	679,304	315,362
Fair Value, Recurring | Significant Other Observable Inputs (Level 2)
Recurring fair value measurements
Total assets at fair value	3,617,625	2,793,179
Fair Value, Recurring | Significant Other Observable Inputs (Level 2) | U.S. Treasury securities
Recurring fair value measurements
Total investments available for sale	0	0
Fair Value, Recurring | Significant Other Observable Inputs (Level 2) | U.S. agency securities
Recurring fair value measurements
Total investments available for sale	18,085	33,605
Fair Value, Recurring | Significant Other Observable Inputs (Level 2) | U.S. agency mortgage-backed securities
Recurring fair value measurements
Total investments available for sale	995,905	848,334
Fair Value, Recurring | Significant Other Observable Inputs (Level 2) | Municipal debt securities
Recurring fair value measurements
Total investments available for sale	551,517	361,638
Fair Value, Recurring | Significant Other Observable Inputs (Level 2) | Non-U.S. government securities
Recurring fair value measurements
Total investments available for sale	61,607	54,995
Fair Value, Recurring | Significant Other Observable Inputs (Level 2) | Corporate debt securities
Recurring fair value measurements
Total investments available for sale	1,126,512	880,301
Fair Value, Recurring | Significant Other Observable Inputs (Level 2) | Residential and commercial mortgage securities
Recurring fair value measurements
Total investments available for sale	409,282	288,281
Fair Value, Recurring | Significant Other Observable Inputs (Level 2) | Asset-backed securities
Recurring fair value measurements
Total investments available for sale	454,717	326,025
Fair Value, Recurring | Significant Other Observable Inputs (Level 2) | Money market funds
Recurring fair value measurements
Total investments available for sale	0	0
Fair Value, Recurring | Significant Unobservable Inputs (Level 3)
Recurring fair value measurements
Total assets at fair value	0	0
Fair Value, Recurring | Significant Unobservable Inputs (Level 3) | U.S. Treasury securities
Recurring fair value measurements
Total investments available for sale	0	0
Fair Value, Recurring | Significant Unobservable Inputs (Level 3) | U.S. agency securities
Recurring fair value measurements
Total investments available for sale	0	0
Fair Value, Recurring | Significant Unobservable Inputs (Level 3) | U.S. agency mortgage-backed securities
Recurring fair value measurements
Total investments available for sale	0	0
Fair Value, Recurring | Significant Unobservable Inputs (Level 3) | Municipal debt securities
Recurring fair value measurements
Total investments available for sale	0	0
Fair Value, Recurring | Significant Unobservable Inputs (Level 3) | Non-U.S. government securities
Recurring fair value measurements
Total investments available for sale	0	0
Fair Value, Recurring | Significant Unobservable Inputs (Level 3) | Corporate debt securities
Recurring fair value measurements
Total investments available for sale	0	0
Fair Value, Recurring | Significant Unobservable Inputs (Level 3) | Residential and commercial mortgage securities
Recurring fair value measurements
Total investments available for sale	0	0
Fair Value, Recurring | Significant Unobservable Inputs (Level 3) | Asset-backed securities
Recurring fair value measurements
Total investments available for sale	0	0
Fair Value, Recurring | Significant Unobservable Inputs (Level 3) | Money market funds
Recurring fair value measurements
Total investments available for sale	$ 0	$ 0