Fair Value of Financial Instruments (Details) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Recurring fair value measurements
Fair Value	$ 2,305,065,000	$ 1,615,102,000
U.S. Treasury securities
Recurring fair value measurements
Fair Value	227,805,000	191,548,000
Municipal debt securities
Recurring fair value measurements
Fair Value	465,255,000	334,324,000
Corporate debt securities
Recurring fair value measurements
Fair Value	611,728,000	456,357,000
Residential and commercial mortgage securities
Recurring fair value measurements
Fair Value	79,407,000	68,336,000
Asset-backed securities
Recurring fair value measurements
Fair Value	167,922,000	127,172,000
Money market funds
Recurring fair value measurements
Fair Value	263,797,000	102,430,000
U.S. Agency | U.S. agency securities
Recurring fair value measurements
Fair Value	33,114,000	18,441,000
U.S. Agency | U.S. agency mortgage-backed securities
Recurring fair value measurements
Fair Value	456,037,000	316,494,000
Recurring basis
Recurring fair value measurements
Total assets at fair value	2,305,065,000	1,615,102,000
Recurring basis | U.S. Treasury securities
Recurring fair value measurements
Fair Value	227,805,000	191,548,000
Recurring basis | Municipal debt securities
Recurring fair value measurements
Fair Value	465,255,000	334,324,000
Recurring basis | Corporate debt securities
Recurring fair value measurements
Fair Value	611,728,000	456,357,000
Recurring basis | Residential and commercial mortgage securities
Recurring fair value measurements
Fair Value	79,407,000	68,336,000
Recurring basis | Asset-backed securities
Recurring fair value measurements
Fair Value	167,922,000	127,172,000
Recurring basis | Money market funds
Recurring fair value measurements
Fair Value	263,797,000	102,430,000
Recurring basis | U.S. Agency | U.S. agency securities
Recurring fair value measurements
Fair Value	33,114,000	18,441,000
Recurring basis | U.S. Agency | U.S. agency mortgage-backed securities
Recurring fair value measurements
Fair Value	456,037,000	316,494,000
Recurring basis | Quoted Prices in Active Markets for Identical Instruments (Level 1)
Recurring fair value measurements
Total assets at fair value	491,602,000	293,978,000
Recurring basis | Quoted Prices in Active Markets for Identical Instruments (Level 1) | U.S. Treasury securities
Recurring fair value measurements
Fair Value	227,805,000	191,548,000
Recurring basis | Quoted Prices in Active Markets for Identical Instruments (Level 1) | Municipal debt securities
Recurring fair value measurements
Fair Value	0	0
Recurring basis | Quoted Prices in Active Markets for Identical Instruments (Level 1) | Corporate debt securities
Recurring fair value measurements
Fair Value	0	0
Recurring basis | Quoted Prices in Active Markets for Identical Instruments (Level 1) | Residential and commercial mortgage securities
Recurring fair value measurements
Fair Value	0	0
Recurring basis | Quoted Prices in Active Markets for Identical Instruments (Level 1) | Asset-backed securities
Recurring fair value measurements
Fair Value	0	0
Recurring basis | Quoted Prices in Active Markets for Identical Instruments (Level 1) | Money market funds
Recurring fair value measurements
Fair Value	263,797,000	102,430,000
Recurring basis | Quoted Prices in Active Markets for Identical Instruments (Level 1) | U.S. Agency | U.S. agency securities
Recurring fair value measurements
Fair Value	0	0
Recurring basis | Quoted Prices in Active Markets for Identical Instruments (Level 1) | U.S. Agency | U.S. agency mortgage-backed securities
Recurring fair value measurements
Fair Value	0	0
Recurring basis | Significant Other Observable Inputs (Level 2)
Recurring fair value measurements
Total assets at fair value	1,813,463,000	1,321,124,000
Recurring basis | Significant Other Observable Inputs (Level 2) | U.S. Treasury securities
Recurring fair value measurements
Fair Value	0	0
Recurring basis | Significant Other Observable Inputs (Level 2) | Municipal debt securities
Recurring fair value measurements
Fair Value	465,255,000	334,324,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Corporate debt securities
Recurring fair value measurements
Fair Value	611,728,000	456,357,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Residential and commercial mortgage securities
Recurring fair value measurements
Fair Value	79,407,000	68,336,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Asset-backed securities
Recurring fair value measurements
Fair Value	167,922,000	127,172,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Money market funds
Recurring fair value measurements
Fair Value	0	0
Recurring basis | Significant Other Observable Inputs (Level 2) | U.S. Agency | U.S. agency securities
Recurring fair value measurements
Fair Value	33,114,000	18,441,000
Recurring basis | Significant Other Observable Inputs (Level 2) | U.S. Agency | U.S. agency mortgage-backed securities
Recurring fair value measurements
Fair Value	456,037,000	316,494,000
Recurring basis | Significant Unobservable Inputs (Level 3)
Recurring fair value measurements
Total assets at fair value	0	0
Recurring basis | Significant Unobservable Inputs (Level 3) | U.S. Treasury securities
Recurring fair value measurements
Fair Value	0	0
Recurring basis | Significant Unobservable Inputs (Level 3) | Municipal debt securities
Recurring fair value measurements
Fair Value	0	0
Recurring basis | Significant Unobservable Inputs (Level 3) | Corporate debt securities
Recurring fair value measurements
Fair Value	0	0
Recurring basis | Significant Unobservable Inputs (Level 3) | Residential and commercial mortgage securities
Recurring fair value measurements
Fair Value	0	0
Recurring basis | Significant Unobservable Inputs (Level 3) | Asset-backed securities
Recurring fair value measurements
Fair Value	0	0
Recurring basis | Significant Unobservable Inputs (Level 3) | Money market funds
Recurring fair value measurements
Fair Value	0	0
Recurring basis | Significant Unobservable Inputs (Level 3) | U.S. Agency | U.S. agency securities
Recurring fair value measurements
Fair Value	0	0
Recurring basis | Significant Unobservable Inputs (Level 3) | U.S. Agency | U.S. agency mortgage-backed securities
Recurring fair value measurements
Fair Value	$ 0	$ 0