Investments (Schedule Of Amortized Cost And Estimated Fair Value Of Fixed Maturities By Contractual Maturity) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Due in one year or less, amortized cost	$ 377,745
Due after one year through five years, amortized cost	1,293,384
Due after five years through ten years, amortized cost	2,103,596
Due after ten years, amortized cost	3,578,361
Amortized cost, sub-total	7,353,086
Fixed maturities, amortized cost	9,950,773	$ 9,551,153
Due in one year or less, estimated fair value	376,564
Due after one year through five years, estimated fair value	1,298,995
Due after five years through ten years, estimated fair value	2,137,866
Due after ten years, estimated fair value	3,679,984
Estimated fair value, sub-total	7,493,409
Available-for-sale fixed maturity investments, estimated fair value	10,043,188	9,940,670
Commercial Mortgage-Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized cost	1,691,572
Fixed maturities, amortized cost	1,691,572	1,244,777
Estimated fair value	1,650,199
Available-for-sale fixed maturity investments, estimated fair value	1,650,199	1,234,326
Residential Mortgage-Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized cost	886,501
Fixed maturities, amortized cost	886,501	846,916
Estimated fair value	880,172
Available-for-sale fixed maturity investments, estimated fair value	880,172	856,168
Asset-Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized cost	19,614
Fixed maturities, amortized cost	19,614	34,942
Estimated fair value	19,408
Available-for-sale fixed maturity investments, estimated fair value	$ 19,408	$ 34,728