Realized and Unrealized Investment Gains (Losses) (Realized and unrealized investmetns gains(losses)) (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended	9 Months Ended
Sep. 30, 2017	Sep. 30, 2016	Jun. 30, 2017	Sep. 30, 2017	Sep. 30, 2016
Realized and Unrealized Investment Gains Losses [Line Items]
Fixed maturity securities, gains	$ 8,763	$ 33,798		$ 21,795	$ 66,972
Fixed maturity securities, losses	(197)	(1,150)		(4,162)	(5,570)
Equity securities available for sale	34,720	8,441		109,566	13,037
Investment funds (1)	124,228	(3,788)		125,383	(9,041)
Real estate	1,956	687		4,892	5,247
Other (2)	14,489	137,750		19,286	136,863
Net realized gains on investments sales	183,959	175,738		276,760	207,508
Other-than-temporary impairments	0	0		0	(18,114)
Net investment gains	183,959	175,738		276,760	189,394
Income tax expense	(64,386)	(61,508)		(96,866)	(66,288)
After-tax net realized investment gains	119,573	114,230		179,894	123,106
Office Building in Washington, D.C.
Realized and Unrealized Investment Gains Losses [Line Items]
Investment funds (1)	$ 124,300			$ 124,300
Aero Precision Industries and Certain Related Aviation Services Business
Realized and Unrealized Investment Gains Losses [Line Items]
Other (2)		$ 134,900			$ 134,900
Common stock
Realized and Unrealized Investment Gains Losses [Line Items]
Other-than-temporary impairments			$ (18,100)

Amount of gain (loss) on sale or disposal of an equity method investment.

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- Definition

This item represents the net total realized and unrealized gain (loss) included in earnings for the period as a result of selling or holding marketable securities categorized as trading, available-for-sale, or held-to-maturity, including the unrealized holding gain (loss) of held-to-maturity securities transferred to the trading security category and the cumulative unrealized gain (loss) which was included in other comprehensive income (a separate component of shareholders' equity) for available-for-sale securities transferred to trading securities during the period. Additionally, this item would include any gains (losses) realized during the period from the sale of investments accounted for under the cost method of accounting and losses recognized for other than temporary impairments (OTTI) of the subject investments.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 9
-Subparagraph (c)-(e)
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27357-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-URI http://asc.fasb.org/extlink&oid=66902209&loc=d3e3602-108585

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.7(c),9(a))
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 10
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27405-111563

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- Definition

Amount of gain (loss) included in earnings for investments classified as other.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=66902209&loc=d3e3602-108585

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- Definition

The amount of other than temporary impairment (OTTI) losses on equity securities, OTTI related to credit losses on debt securities, and OTTI losses on debt securities when the entity intends to sell the securities or it is more likely than not that the entity will be required to sell the securities before recovery of its amortized cost basis. Additionally, this item includes OTTI losses recognized during the period on investments accounted for under the cost method of accounting.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 8B
-URI http://asc.fasb.org/extlink&oid=73527792&loc=SL6284393-111563

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- Definition

Available for Sale and Held-to-Maturity Realized Gains, Debt Securities, Excluding Other than Temporary Impairments and Allowances

+ References

No definition available.

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- Definition

Available for Sale and Held-to-Maturity Realized Losses, Debt Securities, Excluding Other than Temporary Impairments and Allowances

+ References

No definition available.

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- Definition

Available-for-sale Securities, Gross Realized Gain (Loss), Equity Securities, Excluding Other than Temporary Impairments

+ References

No definition available.

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- Definition

REal Estate Realized Gain (Loss) on Disposal

+ References

No definition available.

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- Definition

Realized Gain (Loss) on Investments, Including Equity Method Investments, Excluding Other than Temporary Impairments and Allowances

+ References

No definition available.

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- Definition

Realized Gain (Loss) on Marketable Securities, Equity Method Investments, and Other Investments, Net of Tax

+ References

No definition available.

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- Definition

Realized Gain (Loss) on Marketable Securities, Equity Method Investments, and Other Investments, Tax

+ References

No definition available.

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- Definition

Realized and Unrealized Investment Gains Losses [Line Items]

+ References

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