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Investments (Available-For-Sale Investments) (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, available-for-sale amortized cost	$ 9,203,517	$ 9,448,840
Available-for-sale fixed maturity investments, gross unrealized holding gains	871,181	536,488
Available-for-sale fixed maturity investments, gross unrealized holding losses	(14,024)	(14,419)
Available-for-sale, allowance for credit losses	(202)
Available-for-sale fixed maturity investments, estimated fair value	10,060,472	9,970,909
Short-term investments, available-for-sale, amortized cost	931,555	1,194,953
Short-term investments, available-for-sale, gross unrealized holding gains	489	1,355
Short-term investments, available-for-sale, gross unrealized holding losses	(2,772)	(60)
Short-term investments, available-for-sale, allowance for credit losses	0
Short-term investments, available-for-sale, estimated fair value	929,272	1,196,248
Investments, available-for-sale, amortized cost	10,135,072	10,643,793
Investments, available-for-sale, gross unrealized holding gains	871,670	537,843
Investments, available-for-sale, gross unrealized holding losses	(16,796)	(14,479)
Investments, available-for-sale, allowance for credit losses	(202)
Investments, available-for-sale, estimated fair value	10,989,744	11,167,157
U.S. Treasury securities [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, available-for-sale amortized cost	343,467	282,305
Available-for-sale fixed maturity investments, gross unrealized holding gains	11,447	2,883
Available-for-sale fixed maturity investments, gross unrealized holding losses	(19)	(402)
Available-for-sale, allowance for credit losses	0
Available-for-sale fixed maturity investments, estimated fair value	354,895	284,786
U.S. government-sponsored enterprises [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, available-for-sale amortized cost	323,864	318,831
Available-for-sale fixed maturity investments, gross unrealized holding gains	50,944	23,949
Available-for-sale fixed maturity investments, gross unrealized holding losses	0	(200)
Available-for-sale, allowance for credit losses	0
Available-for-sale fixed maturity investments, estimated fair value	374,808	342,580
Obligations of states, municipalities and political subdivisions [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, available-for-sale amortized cost	3,852,301	3,954,779
Available-for-sale fixed maturity investments, gross unrealized holding gains	354,186	235,915
Available-for-sale fixed maturity investments, gross unrealized holding losses	(1,032)	(812)
Available-for-sale, allowance for credit losses	0
Available-for-sale fixed maturity investments, estimated fair value	4,205,455	4,189,882
Foreign governments [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, available-for-sale amortized cost	1,352,962	1,415,639
Available-for-sale fixed maturity investments, gross unrealized holding gains	172,377	135,763
Available-for-sale fixed maturity investments, gross unrealized holding losses	(9,943)	(9,398)
Available-for-sale, allowance for credit losses	0
Available-for-sale fixed maturity investments, estimated fair value	1,515,396	1,542,004
Commercial mortgage-backed securities [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, available-for-sale amortized cost	1,730,795	1,761,777
Available-for-sale fixed maturity investments, gross unrealized holding gains	148,218	57,450
Available-for-sale fixed maturity investments, gross unrealized holding losses	(35)	(1,382)
Available-for-sale, allowance for credit losses	0
Available-for-sale fixed maturity investments, estimated fair value	1,878,978	1,817,845
Residential mortgage-backed securities [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, available-for-sale amortized cost	836,066	855,641
Available-for-sale fixed maturity investments, gross unrealized holding gains	71,299	32,949
Available-for-sale fixed maturity investments, gross unrealized holding losses	(35)	(517)
Available-for-sale, allowance for credit losses	0
Available-for-sale fixed maturity investments, estimated fair value	907,330	888,073
Asset-backed securities [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, available-for-sale amortized cost	8,122	11,042
Available-for-sale fixed maturity investments, gross unrealized holding gains	159	28
Available-for-sale fixed maturity investments, gross unrealized holding losses	0	(22)
Available-for-sale, allowance for credit losses	0
Available-for-sale fixed maturity investments, estimated fair value	8,281	11,048
Corporate bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, available-for-sale amortized cost	755,940	848,826
Available-for-sale fixed maturity investments, gross unrealized holding gains	62,551	47,551
Available-for-sale fixed maturity investments, gross unrealized holding losses	(2,960)	(1,686)
Available-for-sale, allowance for credit losses	(202)
Available-for-sale fixed maturity investments, estimated fair value	$ 815,329	$ 894,691

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