Guarantor Financial Information Cash Flows (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
Cash flows provided by (used in) operating activities:
Net cash provided by (used in) operating activities		$ (238,124)	$ (47,497)
Cash flows provided by (used in) investing activities:
Proceeds from sale of fixed maturities and short-term investments		4,369,199	2,492,005
Proceeds from redemption of fixed maturities and short-term investments		701,404	1,228,356
Proceeds from sale of equity securities		78,716	46,379
Purchases of fixed maturities and short term investments		(4,917,191)	(3,633,007)
Purchases of equity securities		(33,230)	(47,967)
Proceeds from sale of affiliates		244,340	168,237
Purchases of affiliates		(76,014)	(35,926)
Returns of capital from subsidiaries		0
Other, net		(26,782)	23,429
Net cash provided by (used in) investing activities		340,442	241,506
Cash flows provided by (used in) financing activities:
Proceeds from issuance of ordinary shares and exercise of stock options		46,504	23,296
Buybacks of common shares		(1,663)	(200,488)
Employee withholding on share-based compensation		(16,696)	(22,871)
Dividends paid on common shares		0	(58,392)
Return of capital		0
Distributions to non-controlling interests		(15,946)	(25,400)
Contributions from non-controlling interests		439	26
Contingent consideration paid on business combination		(5,000)	0
Deposit liabilities		(71,124)	(7,771)
Net cash provided by (used in) financing activities		(63,486)	(291,600)
Effects of exchange rate changes on foreign currency cash		7,503	30,273
Increase (decrease) in cash, cash equivalents and restricted cash		46,335	(67,318)
Cash, cash equivalents and restricted cash - beginning of year		3,593,451	3,580,492
Cash, cash equivalents and restricted cash - end of period		3,639,786	3,513,174
Consolidating Adjustments and Eliminations
Cash flows provided by (used in) operating activities:
Net cash provided by (used in) operating activities		0	(400,000)
Cash flows provided by (used in) investing activities:
Proceeds from sale of fixed maturities and short-term investments		0	0
Proceeds from redemption of fixed maturities and short-term investments		0	0
Proceeds from sale of equity securities		0	0
Purchases of fixed maturities and short term investments		0	0
Purchases of equity securities		0	0
Proceeds from sale of affiliates		0	0
Purchases of affiliates		0	0
Returns of capital from subsidiaries		(11,000)
Other, net			0
Net cash provided by (used in) investing activities		(11,000)	0
Cash flows provided by (used in) financing activities:
Proceeds from issuance of ordinary shares and exercise of stock options		0	0
Buybacks of common shares		0	0
Employee withholding on share-based compensation		0	0
Dividends paid on common shares			400,000
Return of capital		11,000
Distributions to non-controlling interests		0	0
Contributions from non-controlling interests		0	0
Contingent consideration paid on business combination		0
Deposit liabilities		0	0
Net cash provided by (used in) financing activities		11,000	400,000
Effects of exchange rate changes on foreign currency cash		0	0
Increase (decrease) in cash, cash equivalents and restricted cash		0	0
Cash, cash equivalents and restricted cash - beginning of year		0	0
Cash, cash equivalents and restricted cash - end of period		0	0
XL-Group/XL-Ireland
Cash flows provided by (used in) operating activities:
Net cash provided by (used in) operating activities	[1]	(39,431)	235,642
Cash flows provided by (used in) investing activities:
Proceeds from sale of fixed maturities and short-term investments	[1]	0	0
Proceeds from redemption of fixed maturities and short-term investments	[1]	0	0
Proceeds from sale of equity securities	[1]	0	0
Purchases of fixed maturities and short term investments	[1]	0	0
Purchases of equity securities	[1]	0	0
Proceeds from sale of affiliates	[1]	0	0
Purchases of affiliates	[1]	0	0
Returns of capital from subsidiaries	[1]	11,000
Other, net	[1]	0	0
Net cash provided by (used in) investing activities	[1]	11,000	0
Cash flows provided by (used in) financing activities:
Proceeds from issuance of ordinary shares and exercise of stock options	[1]	46,504	23,296
Buybacks of common shares	[1]	(1,663)	(200,488)
Employee withholding on share-based compensation	[1]	0	0
Dividends paid on common shares	[1]		(58,392)
Return of capital	[1]	0
Distributions to non-controlling interests	[1]	0	0
Contributions from non-controlling interests	[1]	0	0
Contingent consideration paid on business combination		0
Deposit liabilities	[1]	0	0
Net cash provided by (used in) financing activities	[1]	44,841	(235,584)
Effects of exchange rate changes on foreign currency cash	[1]	0	0
Increase (decrease) in cash, cash equivalents and restricted cash	[1]	16,410	58
Cash, cash equivalents and restricted cash - beginning of year	[1]	3,950	1,022
Cash, cash equivalents and restricted cash - end of period	[1]	20,360	1,080
XLIT
Cash flows provided by (used in) operating activities:
Net cash provided by (used in) operating activities		38,440	118,694
Cash flows provided by (used in) investing activities:
Proceeds from sale of fixed maturities and short-term investments		127,615	547
Proceeds from redemption of fixed maturities and short-term investments		37,790	44,453
Proceeds from sale of equity securities		0	677
Purchases of fixed maturities and short term investments		(107,765)	(3,098)
Purchases of equity securities		0	0
Proceeds from sale of affiliates		0	0
Purchases of affiliates		0	0
Returns of capital from subsidiaries		0
Other, net		0	0
Net cash provided by (used in) investing activities		57,640	42,579
Cash flows provided by (used in) financing activities:
Proceeds from issuance of ordinary shares and exercise of stock options		0	0
Buybacks of common shares		0	0
Employee withholding on share-based compensation		0	0
Dividends paid on common shares			0
Return of capital		(11,000)
Distributions to non-controlling interests		(9,820)	(3,701)
Contributions from non-controlling interests		0	0
Contingent consideration paid on business combination		0
Deposit liabilities		0	0
Net cash provided by (used in) financing activities		(20,820)	(3,701)
Effects of exchange rate changes on foreign currency cash		0	0
Increase (decrease) in cash, cash equivalents and restricted cash		75,260	157,572
Cash, cash equivalents and restricted cash - beginning of year		158,688	84,286
Cash, cash equivalents and restricted cash - end of period		233,948	241,858
Other XL Subsidiaries
Cash flows provided by (used in) operating activities:
Net cash provided by (used in) operating activities		(237,133)	(1,833)
Cash flows provided by (used in) investing activities:
Proceeds from sale of fixed maturities and short-term investments		4,241,584	2,491,458
Proceeds from redemption of fixed maturities and short-term investments		663,614	1,183,903
Proceeds from sale of equity securities		78,716	45,702
Purchases of fixed maturities and short term investments		(4,809,426)	(3,629,909)
Purchases of equity securities		(33,230)	(47,967)
Proceeds from sale of affiliates		244,340	168,237
Purchases of affiliates		(76,014)	(35,926)
Returns of capital from subsidiaries		0
Other, net		(26,782)	23,429
Net cash provided by (used in) investing activities		282,802	198,927
Cash flows provided by (used in) financing activities:
Proceeds from issuance of ordinary shares and exercise of stock options		0	0
Buybacks of common shares		0	0
Employee withholding on share-based compensation		(16,696)	(22,871)
Dividends paid on common shares			(400,000)
Return of capital		0
Distributions to non-controlling interests		(6,126)	(21,699)
Contributions from non-controlling interests		439	26
Contingent consideration paid on business combination		(5,000)
Deposit liabilities		(71,124)	(7,771)
Net cash provided by (used in) financing activities		(98,507)	(452,315)
Effects of exchange rate changes on foreign currency cash		7,503	30,273
Increase (decrease) in cash, cash equivalents and restricted cash		(45,335)	(224,948)
Cash, cash equivalents and restricted cash - beginning of year		3,430,813	3,495,184
Cash, cash equivalents and restricted cash - end of period		$ 3,385,478	$ 3,270,236

[1] Includes XL-Ireland, which was the former ultimate parent and is now in liquidation proceedings. The Company expects the net assets of XL-Ireland to be distributed to XL Group upon completion of the liquidation.