Equity (Schedule of Reclassifications out of AOCI) (Details) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other investment related gains (losses), net										$ 210,519	$ 132,926	$ (107,370)
Other than Temporary Impairment Losses, Investments, Portion in Other Comprehensive Loss, before Tax, Including Portion Attributable to Noncontrolling Interest, Available-for-sale Securities										0	(74)	0
Investment income, net of related expenses										2,154,651	1,911,886	1,734,495
Deferred Policy Acquisition Cost, Amortization Expense, Unrealized Investment Gains (Losses)										8,220	3,541	(17,510)
Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), Reclassification Adjustment from AOCI, before Tax										(11,717)	(444)	(337)
Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss), Reclassification Adjustment from AOCI, before Tax										10,587	(4,427)	(4,618)
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Extraordinary Items, Noncontrolling Interest		$ 255,494	$ 340,162	$ 339,315	$ 207,844	$ 295,543	$ 287,600	$ 353,223	$ 107,580	1,142,815	1,043,946	744,795
Income Tax Expense (Benefit)										679,366	(342,503)	(242,629)
Net Income (Loss) Attributable to Parent		$ 1,216,888	$ 227,591	$ 232,190	$ 145,512	$ 190,149	$ 198,719	$ 236,103	$ 76,472	1,822,181	701,443	$ 502,166
Amortization Of Unrealized Pension And Postretirement Benefits [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), Reclassification Adjustment from AOCI, before Tax	[1]									971	(328)
Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss), Reclassification Adjustment from AOCI, before Tax	[1]									(6,374)	(6,150)
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Extraordinary Items, Noncontrolling Interest										(5,403)	(5,822)
Income Tax Expense (Benefit)										1,891	2,038
Net Income (Loss) Attributable to Parent										(3,512)	(3,784)
Unrealized Appreciation Depreciation Of Investments [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other investment related gains (losses), net										25,039	65,798
Other than Temporary Impairment Losses, Investments, Portion in Other Comprehensive Loss, before Tax, Including Portion Attributable to Noncontrolling Interest, Available-for-sale Securities										0	74
Deferred Policy Acquisition Cost, Amortization Expense, Unrealized Investment Gains (Losses)	[2]									8,220	3,541
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Extraordinary Items, Noncontrolling Interest										34,460	69,622
Income Tax Expense (Benefit)										(10,308)	(17,672)
Net Income (Loss) Attributable to Parent										24,152	51,950
Amounts Reclassified From AOCI [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net Income (Loss) Attributable to Parent										20,640	48,166
Interest Rate Options [Member] | Unrealized Gains Losses On Available For Sale Securities [Member] | Unrealized Appreciation Depreciation Of Investments [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	[3]									(79)	0
Currency Swap [Member] | Unrealized Gains Losses On Available For Sale Securities [Member] | Unrealized Appreciation Depreciation Of Investments [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	[3]									380	510
Forward Contracts [Member] | Unrealized Gains Losses On Available For Sale Securities [Member] | Unrealized Appreciation Depreciation Of Investments [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	[3]									$ 900	$ (301)

[1] See Note 10 for information on employee benefit plans.

[2] See Note 8 for information on deferred policy acquisition costs.

[3] See Note 5 for information on cash flow hedges.