Investments (Investment Income Net of Related Expenses) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Major Categories Of Investment Income Net of Expenses [Line Items]
Investment revenue	$ 576,317	$ 507,781	$ 1,646,934	$ 1,468,130
Investment expense	(19,399)	(18,054)	(57,114)	(53,471)
Investment income, net of related expenses	556,918	489,727	1,589,820	1,414,659
Available-for-sale Securities [Member]
Major Categories Of Investment Income Net of Expenses [Line Items]
Investment revenue	359,157	325,089	1,039,392	961,096
Mortgage Loans on Real Estate [Member]
Major Categories Of Investment Income Net of Expenses [Line Items]
Investment revenue	50,040	39,802	138,829	121,494
Policy loans
Major Categories Of Investment Income Net of Expenses [Line Items]
Investment revenue	15,404	15,391	45,870	47,897
Funds withheld at interest
Major Categories Of Investment Income Net of Expenses [Line Items]
Investment revenue	102,144	104,609	327,089	273,482
Short-term Investments [Member]
Major Categories Of Investment Income Net of Expenses [Line Items]
Investment revenue	1,977	1,752	5,266	6,265
Other invested assets
Major Categories Of Investment Income Net of Expenses [Line Items]
Investment revenue	$ 47,595	$ 21,138	$ 90,488	$ 57,896