Investments (Investment Income Net of Related Expenses) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Major Categories Of Investment Income Net Of Expenses [Line Items]
Gross Investment Income, Operating	$ 2,229,031	$ 2,235,362	$ 1,983,975
Investment Income, Investment Expense	(90,506)	(80,711)	(72,089)
Investment income, net of related expenses	2,138,525	2,154,651	1,911,886
Available-for-sale Securities [Member]
Major Categories Of Investment Income Net Of Expenses [Line Items]
Gross Investment Income, Operating	1,528,086	1,401,585	1,285,406
Equity Securities [Member]
Major Categories Of Investment Income Net Of Expenses [Line Items]
Gross Investment Income, Operating	4,343	4,445	12,513
Mortgage Loans on Real Estate [Member]
Major Categories Of Investment Income Net Of Expenses [Line Items]
Gross Investment Income, Operating	214,387	197,755	168,582
Policy Loans [Member]
Major Categories Of Investment Income Net Of Expenses [Line Items]
Gross Investment Income, Operating	59,332	60,617	63,837
Funds Withheld At Interest [Member]
Major Categories Of Investment Income Net Of Expenses [Line Items]
Gross Investment Income, Operating	309,977	457,774	368,728
Short-term Investments [Member]
Major Categories Of Investment Income Net Of Expenses [Line Items]
Gross Investment Income, Operating	14,349	7,171	8,051
Other Aggregated Investments [Member]
Major Categories Of Investment Income Net Of Expenses [Line Items]
Gross Investment Income, Operating	$ 98,557	$ 106,015	$ 76,858