Investments (Investment Income Net of Related Expenses) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Major Categories Of Investment Income Net of Expenses [Line Items]
Investment revenue	$ 608,736	$ 549,734	$ 1,209,610	$ 1,088,116
Investment expense	(24,658)	(21,673)	(45,655)	(43,726)
Investment income, net of related expenses	584,078	528,061	1,163,955	1,044,390
Available-for-sale Securities [Member]
Major Categories Of Investment Income Net of Expenses [Line Items]
Investment revenue	427,841	373,624	842,928	742,827
Equity securities
Major Categories Of Investment Income Net of Expenses [Line Items]
Investment revenue	645	709	1,891	2,391
Mortgage Loans on Real Estate [Member]
Major Categories Of Investment Income Net of Expenses [Line Items]
Investment revenue	60,509	50,460	120,071	100,659
Policy loans
Major Categories Of Investment Income Net of Expenses [Line Items]
Investment revenue	14,394	14,775	28,503	29,555
Funds withheld at interest
Major Categories Of Investment Income Net of Expenses [Line Items]
Investment revenue	65,972	86,417	127,706	161,862
Short-term Investments [Member]
Major Categories Of Investment Income Net of Expenses [Line Items]
Investment revenue	7,465	2,964	14,365	6,209
Other invested assets
Major Categories Of Investment Income Net of Expenses [Line Items]
Investment revenue	$ 31,910	$ 20,785	$ 74,146	$ 44,613