Investments (Investment Income Net of Related Expenses) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Major Categories Of Investment Income Net of Expenses [Line Items]
Investment revenue	$ 537,583	$ 525,229	$ 1,070,617	$ 960,349
Investment expense	(19,045)	(17,563)	(37,715)	(35,417)
Investment income, net of related expenses	518,538	507,666	1,032,902	924,932
Available-for-sale Securities [Member]
Major Categories Of Investment Income Net of Expenses [Line Items]
Investment revenue	355,735	323,592	680,235	636,007
Mortgage Loans on Real Estate [Member]
Major Categories Of Investment Income Net of Expenses [Line Items]
Investment revenue	44,442	41,900	88,789	81,692
Policy loans
Major Categories Of Investment Income Net of Expenses [Line Items]
Investment revenue	15,194	16,372	30,466	32,506
Funds withheld at interest
Major Categories Of Investment Income Net of Expenses [Line Items]
Investment revenue	97,367	112,893	224,945	168,873
Short-term Investments [Member]
Major Categories Of Investment Income Net of Expenses [Line Items]
Investment revenue	1,779	2,322	3,289	4,513
Other invested assets
Major Categories Of Investment Income Net of Expenses [Line Items]
Investment revenue	$ 23,066	$ 28,150	$ 42,893	$ 36,758