Investments (Schedule of Net Realized and Unrealized (Losses) Gains on Investments) (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Gain (Loss) on Investments [Line Items]
Net realized and unrealized (losses) gains on investments	$ (17,901)	$ 58,113	$ (100,045)	$ 101,486
Fixed maturity investments
Gain (Loss) on Investments [Line Items]
Gross realized gains	5,133	15,249	9,716	26,710
Gross realized losses	(26,519)	(7,243)	(52,372)	(23,776)
Net realized (losses) gains on fixed maturity investments	(21,386)	8,006	(42,656)	2,934
Net unrealized (losses) gains on fixed maturity investments trading	(9,420)	18,760	(64,792)	43,395
Derivatives
Gain (Loss) on Investments [Line Items]
Net realized and unrealized gains (losses) on investments-related derivatives	1,038	(268)	(3,326)	(324)
Equity investments
Gain (Loss) on Investments [Line Items]
Net unrealized gains (losses) on investments trading	11,519	16,469	10,147	19,420
Net realized (losses) gains on fixed maturity investments	348	15,146	582	36,061
Net realized and unrealized gains on equity investments trading	$ 11,867	$ 31,615	$ 10,729	$ 55,481

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.

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=82897076&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=98514028&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=82897076&loc=d3e27405-111563

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