Investments, Summary of Available-for-sale Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Available-for-sale Debt Securities [Abstract]
Fair value	$ 914,949	$ 919,692
Cost or amortized cost	901,009	894,468
Allowance for credit losses	(825)	(1,035)
Gross unrealized gains	20,784	30,997
Gross unrealized losses	(6,019)	(4,738)
Net unrealized gains (losses)	14,765	26,259
Agency Collateralized Mortgage Obligations [Member]
Available-for-sale Debt Securities [Abstract]
Fair value	11,821	11,931
Cost or amortized cost	11,405	11,448
Allowance for credit losses	0	0
Gross unrealized gains	485	483
Gross unrealized losses	(69)	0
Net unrealized gains (losses)	416	483
Agency Mortgage-backed Securities [Member]
Available-for-sale Debt Securities [Abstract]
Fair value	114,822	102,107
Cost or amortized cost	113,228	99,060
Allowance for credit losses	0	0
Gross unrealized gains	2,254	3,062
Gross unrealized losses	(660)	(15)
Net unrealized gains (losses)	1,594	3,047
Asset-backed Securities [Member]
Available-for-sale Debt Securities [Abstract]
Fair value	102,638	107,696
Cost or amortized cost	102,799	108,686
Allowance for credit losses	0	0
Gross unrealized gains	540	596
Gross unrealized losses	(701)	(1,586)
Net unrealized gains (losses)	(161)	(990)
Bank Loans [Member]
Available-for-sale Debt Securities [Abstract]
Fair value	12,073	11,361
Cost or amortized cost	11,998	11,590
Allowance for credit losses	0	0
Gross unrealized gains	130	128
Gross unrealized losses	(55)	(357)
Net unrealized gains (losses)	75	(229)
Collateralized Mortgage Obligations [Member]
Available-for-sale Debt Securities [Abstract]
Fair value	6,167	5,118
Cost or amortized cost	6,027	5,061
Allowance for credit losses	0	0
Gross unrealized gains	240	151
Gross unrealized losses	(100)	(94)
Net unrealized gains (losses)	140	57
Corporate Securities [Member]
Available-for-sale Debt Securities [Abstract]
Fair value	362,442	360,241
Cost or amortized cost	353,462	344,059
Allowance for credit losses	0	0
Gross unrealized gains	11,149	16,380
Gross unrealized losses	(2,169)	(198)
Net unrealized gains (losses)	8,980	16,182
Mortgage-backed Securities [Member]
Available-for-sale Debt Securities [Abstract]
Fair value	38,612	38,056
Cost or amortized cost	39,967	40,675
Allowance for credit losses	(825)	(1,035)
Gross unrealized gains	490	659
Gross unrealized losses	(1,020)	(2,243)
Net unrealized gains (losses)	(530)	(1,584)
Municipal Obligations [Member]
Available-for-sale Debt Securities [Abstract]
Fair value	46,464	45,143
Cost or amortized cost	45,358	43,353
Allowance for credit losses	0	0
Gross unrealized gains	1,184	1,820
Gross unrealized losses	(78)	(30)
Net unrealized gains (losses)	1,106	1,790
Non-U.S. Government Obligations [Member]
Available-for-sale Debt Securities [Abstract]
Fair value	30,648	30,600
Cost or amortized cost	30,083	29,882
Allowance for credit losses	0	0
Gross unrealized gains	574	718
Gross unrealized losses	(9)	0
Net unrealized gains (losses)	565	718
U.S. Government Obligations [Member]
Available-for-sale Debt Securities [Abstract]
Fair value	189,262	207,439
Cost or amortized cost	186,682	200,654
Allowance for credit losses	0	0
Gross unrealized gains	3,738	7,000
Gross unrealized losses	(1,158)	(215)
Net unrealized gains (losses)	$ 2,580	$ 6,785

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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