Investments (Investment Income Net of Related Expenses) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Major Categories Of Investment Income Net of Expenses [Line Items]
Investment revenue	$ 702,416	$ 596,035	$ 1,912,026	$ 1,684,151
Investment expense	(23,611)	(23,293)	(69,266)	(67,019)
Investment income, net of related expenses	678,805	572,742	1,842,760	1,617,132
Available-for-sale Securities [Member]
Major Categories Of Investment Income Net of Expenses [Line Items]
Investment revenue	464,431	378,669	1,307,359	1,121,496
Equity securities
Major Categories Of Investment Income Net of Expenses [Line Items]
Investment revenue	1,709	1,319	3,600	3,710
Mortgage Loans on Real Estate [Member]
Major Categories Of Investment Income Net of Expenses [Line Items]
Investment revenue	67,071	54,424	187,142	155,083
Policy loans
Major Categories Of Investment Income Net of Expenses [Line Items]
Investment revenue	14,580	14,730	43,083	44,285
Funds withheld at interest
Major Categories Of Investment Income Net of Expenses [Line Items]
Investment revenue	81,862	108,232	209,568	270,094
Short-term Investments [Member]
Major Categories Of Investment Income Net of Expenses [Line Items]
Investment revenue	6,152	3,067	20,517	9,276
Other invested assets
Major Categories Of Investment Income Net of Expenses [Line Items]
Investment revenue	$ 66,611	$ 35,594	$ 140,757	$ 80,207