Fair Value of Financial Instruments - Fair Value Hierarchy (Details) $ in Thousands	Mar. 31, 2019 USD ($) security	Dec. 31, 2018 USD ($) security
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities, available-for-sale, Fair value	$ 3,131,934	$ 3,051,568
Assets, number of securities priced for fair value | security	1	1
U.S. treasury bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities, available-for-sale, Fair value	$ 116,662	$ 139,072
U.S. agency bonds – mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities, available-for-sale, Fair value	1,063,322	1,453,134
U.S. agency bonds – other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities, available-for-sale, Fair value	19,780	129,233
Non-U.S. government and supranational bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities, available-for-sale, Fair value	33,220	10,072
Asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities, available-for-sale, Fair value	90,365	215,082
Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities, available-for-sale, Fair value	1,751,694	1,104,975
Municipal bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities, available-for-sale, Fair value	56,891
Significant Other Observable Inputs (Level 2) | Fixed maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, securities valued using market approach	5,484	5,676
Fair Value, Measurements, Recurring | Total Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Based on NAV Practical Expedient	24,693	23,716
Total	$ 3,156,627	$ 3,075,284
Percentage of total assets	69.20%	58.20%
Fair Value, Measurements, Recurring | Total Fair Value | Fixed maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, valued by third party, percentage	99.80%	99.90%
Assets, valued using market approach, percentage	0.20%	0.10%
Fair Value, Measurements, Recurring | Total Fair Value | U.S. treasury bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities, available-for-sale, Fair value	$ 116,662	$ 139,072
Fair Value, Measurements, Recurring | Total Fair Value | U.S. agency bonds – mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities, available-for-sale, Fair value	1,063,322	1,453,134
Fair Value, Measurements, Recurring | Total Fair Value | U.S. agency bonds – other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities, available-for-sale, Fair value	19,780	129,233
Fair Value, Measurements, Recurring | Total Fair Value | Non-U.S. government and supranational bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities, available-for-sale, Fair value	33,220	10,072
Fair Value, Measurements, Recurring | Total Fair Value | Asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities, available-for-sale, Fair value	90,365	215,082
Fair Value, Measurements, Recurring | Total Fair Value | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities, available-for-sale, Fair value	1,751,694	1,104,975
Fair Value, Measurements, Recurring | Total Fair Value | Municipal bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities, available-for-sale, Fair value	56,891
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Based on NAV Practical Expedient	0	0
Total	$ 116,662	$ 139,072
Percentage of total assets	2.50%	2.60%
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. treasury bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities, available-for-sale, Fair value	$ 116,662	$ 139,072
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. agency bonds – mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities, available-for-sale, Fair value	0	0
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. agency bonds – other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities, available-for-sale, Fair value	0	0
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Non-U.S. government and supranational bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities, available-for-sale, Fair value	0	0
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities, available-for-sale, Fair value	0	0
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities, available-for-sale, Fair value	0	0
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Municipal bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities, available-for-sale, Fair value	0
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Based on NAV Practical Expedient	0	0
Total	$ 3,015,272	$ 2,912,496
Percentage of total assets	66.10%	55.10%
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | U.S. treasury bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities, available-for-sale, Fair value	$ 0	$ 0
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | U.S. agency bonds – mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities, available-for-sale, Fair value	1,063,322	1,453,134
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | U.S. agency bonds – other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities, available-for-sale, Fair value	19,780	129,233
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Non-U.S. government and supranational bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities, available-for-sale, Fair value	33,220	10,072
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities, available-for-sale, Fair value	90,365	215,082
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities, available-for-sale, Fair value	1,751,694	1,104,975
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Municipal bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities, available-for-sale, Fair value	56,891
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Based on NAV Practical Expedient	21,412	19,883
Total	$ 21,412	$ 19,883
Percentage of total assets	0.50%	0.40%
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | U.S. treasury bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities, available-for-sale, Fair value	$ 0	$ 0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | U.S. agency bonds – mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities, available-for-sale, Fair value	0	0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | U.S. agency bonds – other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities, available-for-sale, Fair value	0	0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Non-U.S. government and supranational bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities, available-for-sale, Fair value	0	0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities, available-for-sale, Fair value	0	0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities, available-for-sale, Fair value	0	0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Municipal bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed Maturities, available-for-sale, Fair value	0
Fair Value, Measurements, Recurring | Fair Value Based on NAV Practical Expedient
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Based on NAV Practical Expedient	$ 3,281	$ 3,833
Percentage of total assets	0.10%	0.10%