Investments (Other Investment Portfolio by Strategy and the Remaining Commitment Amount and Maximum Exposure to Loss Associated With Each Strategy) (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Net investment income earned		$ 68,185	$ 55,826	$ 158,596	$ 164,949
Carrying Value		258,648		258,648		$ 216,807
Alternative Investments [Member]
Net investment income earned		18,700	$ 5,200	8,900	$ 13,000
Other Securities [Member]
Carrying Value		40,725		40,725		32,667
Remaining Commitment		0		0		0
Maximum Exposure to Loss	[1]	40,725		40,725		32,667
Alternative Investments [Member]
Carrying Value		217,923		217,923		184,140
Remaining Commitment		225,374		225,374		219,219
Maximum Exposure to Loss	[1]	443,297		443,297		403,359
Alternative Investments [Member] | Private Equity Funds [Member]
Carrying Value		146,810		146,810		118,352
Remaining Commitment		110,173		110,173		93,138
Maximum Exposure to Loss	[1]	256,983		256,983		211,490
Alternative Investments [Member] | Private Credit Funds [Member]
Carrying Value		49,254		49,254		42,532
Remaining Commitment		96,131		96,131		105,340
Maximum Exposure to Loss	[1]	145,385		145,385		147,872
Alternative Investments [Member] | Real Assets [Member]
Carrying Value		21,859		21,859		23,256
Remaining Commitment		19,070		19,070		20,741
Maximum Exposure to Loss	[1]	40,929		40,929		43,997
Other Investments [Member]
Carrying Value		258,648		258,648		216,807
Remaining Commitment		225,374		225,374		219,219
Maximum Exposure to Loss	[1]	$ 484,022		$ 484,022		$ 436,026

[1] The maximum exposure to loss includes both the carry value of these investments and the related remaining commitments. In addition, tax credits that have been previously recognized in Other securities are subject to the risk of recapture, which we do not consider significant.