Investments (Investment Income Net of Related Expenses) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Major Categories Of Investment Income Net of Expenses [Line Items]
Investment revenue	$ 549,734	$ 537,583	$ 1,088,116	$ 1,070,617
Investment expense	(21,673)	(19,045)	(43,726)	(37,715)
Investment income, net of related expenses	528,061	518,538	1,044,390	1,032,902
Available-for-sale Securities [Member]
Major Categories Of Investment Income Net of Expenses [Line Items]
Investment revenue	373,624	355,735	742,827	680,235
Equity securities
Major Categories Of Investment Income Net of Expenses [Line Items]
Investment revenue	709	995	2,391	2,354
Mortgage Loans on Real Estate [Member]
Major Categories Of Investment Income Net of Expenses [Line Items]
Investment revenue	50,460	44,442	100,659	88,789
Policy loans
Major Categories Of Investment Income Net of Expenses [Line Items]
Investment revenue	14,775	15,194	29,555	30,466
Funds withheld at interest
Major Categories Of Investment Income Net of Expenses [Line Items]
Investment revenue	86,417	97,367	161,862	224,945
Short-term Investments [Member]
Major Categories Of Investment Income Net of Expenses [Line Items]
Investment revenue	2,964	1,779	6,209	3,289
Other invested assets
Major Categories Of Investment Income Net of Expenses [Line Items]
Investment revenue	$ 20,785	$ 22,071	$ 44,613	$ 40,539