Segment Reporting Disclosures (Company's Segment Disclosures) (Details) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) segment	Jun. 30, 2016 USD ($)
Segment Reporting Information [Line Items]
Number of reportable ongoing underwriting segments | segment				3
Gross premium volume		$ 1,357,164	$ 1,277,649	$ 2,817,915	$ 2,670,312
Net written premiums		1,137,657	1,050,546	2,397,886	2,232,506
Earned premiums		1,033,574	950,859	2,016,176	1,908,545
Losses and loss adjustment expenses, current accident year		(684,585)	(651,238)	(1,322,931)	(1,243,523)
Losses and loss adjustment expenses, prior accident years		161,607	139,682	188,234	258,003
Amortization of policy acquisition costs		(213,474)	(188,366)	(418,022)	(378,335)
Other operating expenses		(186,561)	(187,214)	(355,244)	(361,933)
Underwriting profit (loss)		110,561	63,723	108,213	182,757
Net investment income		99,299	94,996	199,667	186,290
Net realized investment gains		17,627	17,241	38,492	38,420
Other revenues (insurance)		2,388	2,013	7,898	8,048
Other expenses (insurance)		(9,982)	(10,020)	(20,150)	(20,299)
Segment profit (loss)		219,893	167,953	334,120	395,216
Other revenues (non-insurance)		328,605	310,828	631,011	610,816
Other expenses (non-insurance)		(289,130)	(267,889)	(561,547)	(532,703)
Amortization of intangible assets		(18,026)	(17,204)	(34,796)	(34,464)
Interest expense		(31,797)	(33,697)	(65,199)	(64,538)
Loss on early extinguishment of debt		0	(44,100)	0	(44,100)
Income Before Income Taxes		$ 209,545	$ 115,891	$ 303,589	$ 330,227
U.S. GAAP combined ratio	[1]	89.00%	93.00%	95.00%	90.00%
U.S. Insurance [Member]
Segment Reporting Information [Line Items]
Gross premium volume		$ 753,329	$ 689,468	$ 1,393,158	$ 1,337,258
Net written premiums		630,453	579,233	1,175,558	1,131,978
Earned premiums		578,241	533,328	1,127,577	1,065,796
Losses and loss adjustment expenses, current accident year		(379,809)	(352,092)	(726,115)	(668,425)
Losses and loss adjustment expenses, prior accident years		77,266	66,332	119,886	104,986
Amortization of policy acquisition costs		(124,032)	(112,585)	(236,998)	(220,589)
Other operating expenses		(108,684)	(100,330)	(202,059)	(189,789)
Underwriting profit (loss)		42,982	34,653	82,291	91,979
Net investment income		0	0	0	0
Net realized investment gains		0	0	0	0
Other revenues (insurance)		1,043	958	1,706	2,377
Other expenses (insurance)		(85)	(684)	(843)	(1,408)
Segment profit (loss)		$ 43,940	$ 34,927	$ 83,154	$ 92,948
U.S. GAAP combined ratio	[1]	93.00%	94.00%	93.00%	91.00%
International Insurance [Member]
Segment Reporting Information [Line Items]
Gross premium volume		$ 355,949	$ 318,581	$ 629,117	$ 609,985
Net written premiums		286,833	244,636	512,245	471,035
Earned premiums		225,948	203,052	433,461	418,397
Losses and loss adjustment expenses, current accident year		(158,590)	(146,453)	(305,020)	(291,929)
Losses and loss adjustment expenses, prior accident years		55,262	39,002	105,528	68,654
Amortization of policy acquisition costs		(36,356)	(32,873)	(71,079)	(67,145)
Other operating expenses		(54,203)	(63,689)	(106,478)	(118,023)
Underwriting profit (loss)		32,061	(961)	56,412	9,954
Net investment income		0	0	0	0
Net realized investment gains		0	0	0	0
Other revenues (insurance)		574	609	4,569	4,730
Other expenses (insurance)		(2,728)	(2,137)	(5,074)	(3,691)
Segment profit (loss)		$ 29,907	$ (2,489)	$ 55,907	$ 10,993
U.S. GAAP combined ratio	[1]	86.00%	100.00%	87.00%	98.00%
Reinsurance [Member]
Segment Reporting Information [Line Items]
Gross premium volume		$ 247,902	$ 269,604	$ 795,639	$ 723,090
Net written premiums		220,466	226,681	710,062	629,407
Earned premiums		229,480	214,514	455,117	424,133
Losses and loss adjustment expenses, current accident year		(146,186)	(152,693)	(291,796)	(283,169)
Losses and loss adjustment expenses, prior accident years		28,151	34,644	(43,412)	71,005
Amortization of policy acquisition costs		(53,086)	(42,908)	(109,945)	(90,601)
Other operating expenses		(23,539)	(23,235)	(46,408)	(54,047)
Underwriting profit (loss)		34,820	30,322	(36,444)	67,321
Net investment income		0	0	0	0
Net realized investment gains		0	0	0	0
Other revenues (insurance)		0	0	416	0
Other expenses (insurance)		0	0	0	0
Segment profit (loss)		$ 34,820	$ 30,322	$ (36,028)	$ 67,321
U.S. GAAP combined ratio	[1]	85.00%	86.00%	108.00%	84.00%
Other Insurance (Discontinued Lines) [Member]
Segment Reporting Information [Line Items]
Gross premium volume		$ (16)	$ (4)	$ 1	$ (21)
Net written premiums		(95)	(4)	21	86
Earned premiums		(95)	(35)	21	219
Losses and loss adjustment expenses, current accident year		0	0	0	0
Losses and loss adjustment expenses, prior accident years		928	(296)	6,232	13,358
Amortization of policy acquisition costs		0	0	0	0
Other operating expenses		(135)	40	(299)	(74)
Underwriting profit (loss)		698	(291)	5,954	13,503
Net investment income		0	0	0	0
Net realized investment gains		0	0	0	0
Other revenues (insurance)		771	446	1,207	941
Other expenses (insurance)		(7,169)	(7,199)	(14,233)	(15,200)
Segment profit (loss)		$ (5,700)	$ (7,044)	$ (7,072)	$ (756)
U.S. GAAP combined ratio - not meaniningful	[1],[2]	NM	NM	NM	NM
Investing [Member]
Segment Reporting Information [Line Items]
Gross premium volume		$ 0	$ 0	$ 0	$ 0
Net written premiums		0	0	0	0
Earned premiums		0	0	0	0
Losses and loss adjustment expenses, current accident year		0	0	0	0
Losses and loss adjustment expenses, prior accident years		0	0	0	0
Amortization of policy acquisition costs		0	0	0	0
Other operating expenses		0	0	0	0
Underwriting profit (loss)		0	0	0	0
Net investment income		99,299	94,996	199,667	186,290
Net realized investment gains		17,627	17,241	38,492	38,420
Other revenues (insurance)		0	0	0	0
Other expenses (insurance)		0	0	0	0
Segment profit (loss)		$ 116,926	$ 112,237	$ 238,159	$ 224,710

[1] The U.S. GAAP combined ratio is a measure of underwriting performance and represents the relationship of incurred losses, loss adjustment expenses and underwriting, acquisition and insurance expenses to earned premiums.

[2] NM – Ratio is not meaningful.