Investments (Investment Income Net of Related Expenses) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Major Categories Of Investment Income Net Of Expenses [Line Items]
Gross Investment Income, Operating	$ 1,983,975	$ 1,802,539	$ 1,776,011
Investment Income, Investment Expense	(72,089)	(68,044)	(62,320)
Investment income, net of related expenses	1,911,886	1,734,495	1,713,691
Available-for-sale Securities [Member]
Major Categories Of Investment Income Net Of Expenses [Line Items]
Gross Investment Income, Operating	1,285,406	1,177,706	1,052,715
Mortgage Loans on Real Estate [Member]
Major Categories Of Investment Income Net Of Expenses [Line Items]
Gross Investment Income, Operating	168,582	149,564	148,417
Policy Loans [Member]
Major Categories Of Investment Income Net Of Expenses [Line Items]
Gross Investment Income, Operating	63,837	62,955	55,248
Funds Withheld At Interest [Member]
Major Categories Of Investment Income Net Of Expenses [Line Items]
Gross Investment Income, Operating	368,728	343,031	447,364
Short-term Investments [Member]
Major Categories Of Investment Income Net Of Expenses [Line Items]
Gross Investment Income, Operating	8,051	7,574	8,955
Other Aggregated Investments [Member]
Major Categories Of Investment Income Net Of Expenses [Line Items]
Gross Investment Income, Operating	$ 89,371	$ 61,709	$ 63,312