Investments (Investment Income Net of Related Expenses) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Major Categories Of Investment Income Net Of Expenses [Line Items]
Gross Investment Income, Operating	$ 2,235,362	$ 1,983,975	$ 1,802,539
Investment Income, Investment Expense	(80,711)	(72,089)	(68,044)
Investment income, net of related expenses	2,154,651	1,911,886	1,734,495
Available-for-sale Securities [Member]
Major Categories Of Investment Income Net Of Expenses [Line Items]
Gross Investment Income, Operating	1,401,585	1,285,406	1,177,706
Mortgage Loans on Real Estate [Member]
Major Categories Of Investment Income Net Of Expenses [Line Items]
Gross Investment Income, Operating	197,755	168,582	149,564
Policy Loans [Member]
Major Categories Of Investment Income Net Of Expenses [Line Items]
Gross Investment Income, Operating	60,617	63,837	62,955
Funds Withheld At Interest [Member]
Major Categories Of Investment Income Net Of Expenses [Line Items]
Gross Investment Income, Operating	457,774	368,728	343,031
Short-term Investments [Member]
Major Categories Of Investment Income Net Of Expenses [Line Items]
Gross Investment Income, Operating	7,171	8,051	7,574
Other Aggregated Investments [Member]
Major Categories Of Investment Income Net Of Expenses [Line Items]
Gross Investment Income, Operating	$ 110,460	$ 89,371	$ 61,709