Segment Reporting Disclosures (Company's Segment Disclosures) (Details) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2016 USD ($)	Jun. 30, 2015 USD ($)	Jun. 30, 2016 USD ($) segment	Jun. 30, 2015 USD ($)
Segment Reporting Information [Line Items]
Number of reportable ongoing underwriting segments | segment				3
Gross premium volume		$ 1,277,649	$ 1,265,786	$ 2,670,312	$ 2,518,259
Net written premiums		1,050,546	1,034,344	2,232,506	2,069,774
Earned premiums		950,859	957,557	1,908,545	1,901,207
Losses and loss adjustment expenses, current accident year		(651,238)	(664,854)	(1,243,523)	(1,279,195)
Losses and loss adjustment expenses, prior accident years		139,682	128,660	258,003	296,006
Underwriting, acquisition and insurance expenses		(375,580)	(379,652)	(740,268)	(720,337)
Underwriting profit (loss)		63,723	41,711	182,757	197,681
Net investment income		94,996	90,586	186,290	183,461
Net realized investment gains		17,241	6,105	38,420	11,676
Other revenues (insurance)		2,013	3,937	8,048	11,126
Other expenses (insurance)		(10,020)	(5,950)	(20,299)	(15,608)
Segment profit (loss)		167,953	136,389	395,216	388,336
Other revenues (non-insurance)		310,828	246,420	610,816	499,289
Other expenses (non-insurance)		(267,889)	(236,286)	(532,703)	(457,629)
Amortization of intangible assets		(17,204)	(16,949)	(34,464)	(31,589)
Interest expense		(33,697)	(29,288)	(64,538)	(58,600)
Loss on early extinguishment of debt		(44,100)	0	(44,100)	0
Income Before Income Taxes		$ 115,891	$ 100,286	$ 330,227	$ 339,807
U.S. GAAP combined ratio	[1]	93.00%	96.00%	90.00%	90.00%
U.S. Insurance [Member]
Segment Reporting Information [Line Items]
Gross premium volume		$ 689,468	$ 668,853	$ 1,337,258	$ 1,254,218
Net written premiums		579,233	554,638	1,131,978	1,050,807
Earned premiums		533,328	520,446	1,065,796	1,035,000
Losses and loss adjustment expenses, current accident year		(352,092)	(341,335)	(668,425)	(658,092)
Losses and loss adjustment expenses, prior accident years		66,332	68,620	104,986	136,201
Underwriting, acquisition and insurance expenses		(212,915)	(211,856)	(410,378)	(397,116)
Underwriting profit (loss)		34,653	35,875	91,979	115,993
Net investment income		0	0	0	0
Net realized investment gains		0	0	0	0
Other revenues (insurance)		958	2,203	2,377	3,605
Other expenses (insurance)		(684)	(1,284)	(1,408)	(2,189)
Segment profit (loss)		$ 34,927	$ 36,794	$ 92,948	$ 117,409
U.S. GAAP combined ratio	[1]	94.00%	93.00%	91.00%	89.00%
International Insurance [Member]
Segment Reporting Information [Line Items]
Gross premium volume		$ 318,581	$ 338,159	$ 609,985	$ 627,386
Net written premiums		244,636	264,129	471,035	486,837
Earned premiums		203,052	223,941	418,397	429,902
Losses and loss adjustment expenses, current accident year		(146,453)	(168,896)	(291,929)	(317,740)
Losses and loss adjustment expenses, prior accident years		39,002	43,373	68,654	120,023
Underwriting, acquisition and insurance expenses		(96,562)	(94,617)	(185,168)	(173,411)
Underwriting profit (loss)		(961)	3,801	9,954	58,774
Net investment income		0	0	0	0
Net realized investment gains		0	0	0	0
Other revenues (insurance)		609	915	4,730	6,302
Other expenses (insurance)		(2,137)	(1,318)	(3,691)	(2,722)
Segment profit (loss)		$ (2,489)	$ 3,398	$ 10,993	$ 62,354
U.S. GAAP combined ratio	[1]	100.00%	98.00%	98.00%	86.00%
Reinsurance [Member]
Segment Reporting Information [Line Items]
Gross premium volume		$ 269,604	$ 258,745	$ 723,090	$ 636,582
Net written premiums		226,681	215,520	629,407	531,732
Earned premiums		214,514	213,140	424,133	435,894
Losses and loss adjustment expenses, current accident year		(152,693)	(154,623)	(283,169)	(303,363)
Losses and loss adjustment expenses, prior accident years		34,644	15,118	71,005	41,505
Underwriting, acquisition and insurance expenses		(66,143)	(73,170)	(144,648)	(149,723)
Underwriting profit (loss)		30,322	465	67,321	24,313
Net investment income		0	0	0	0
Net realized investment gains		0	0	0	0
Other revenues (insurance)		0	469	0	892
Other expenses (insurance)		0	0	0	0
Segment profit (loss)		$ 30,322	$ 934	$ 67,321	$ 25,205
U.S. GAAP combined ratio	[1]	86.00%	100.00%	84.00%	94.00%
Other Insurance (Discontinued Lines) [Member]
Segment Reporting Information [Line Items]
Gross premium volume		$ (4)	$ 29	$ (21)	$ 73
Net written premiums		(4)	57	86	398
Earned premiums		(35)	30	219	411
Losses and loss adjustment expenses, current accident year		0	0	0	0
Losses and loss adjustment expenses, prior accident years		(296)	1,549	13,358	(1,723)
Underwriting, acquisition and insurance expenses		40	(9)	(74)	(87)
Underwriting profit (loss)		(291)	1,570	13,503	(1,399)
Net investment income		0	0	0	0
Net realized investment gains		0	0	0	0
Other revenues (insurance)		446	350	941	327
Other expenses (insurance)		(7,199)	(3,348)	(15,200)	(10,697)
Segment profit (loss)		$ (7,044)	$ (1,428)	$ (756)	$ (11,769)
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
Underwriting, acquisition and insurance expenses		0	0	0	0
Underwriting profit (loss)		0	0	0	0
Net investment income		94,996	90,586	186,290	183,461
Net realized investment gains		17,241	6,105	38,420	11,676
Other revenues (insurance)		0	0	0	0
Other expenses (insurance)		0	0	0	0
Segment profit (loss)		$ 112,237	$ 96,691	$ 224,710	$ 195,137

[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.