Liability for Unpaid Loss and Loss Adjustment Expense (Details) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseNetAbstract
Reserves at beginning of period				$ 3,902,132,000				$ 3,767,850,000	$ 3,902,132,000	$ 3,767,850,000	$ 3,658,317,000
Less reinsurance recoverables on reserves				1,122,731,000				1,018,047,000	1,122,731,000	1,018,047,000	974,834,000
Net reserves at beginning of period				2,779,401,000				2,749,803,000	2,779,401,000	2,749,803,000	2,683,483,000
Net reserve additions from acquired businesses											14,705,000,000
Net loss and loss adjustment expense:
Provision for loss and loss adjustment expense for claims occurring in current year									1,383,192,000	1,396,196,000	1,408,085,000
Increase (decrease) in estimated loss and loss adjustment expense for claims occurring in prior years									(56,357,000)	(73,742,000)	(70,011,000)
Net loss and loss adjustment expense	339,434,000	320,758,000	335,897,000	330,746,000	305,477,000	328,530,000	347,353,000	341,094,000	1,326,835,000	1,322,454,000	1,338,074,000
Net loss and loss adjustment expense payments for claims occurring during:
Current year									543,605,000	562,049,000	575,573,000
Prior years									856,896,000	736,351,000	729,335,000
Net loss and loss adjustment expense payments									1,400,501,000	1,298,400,000	1,304,908,000
Foreign currency adjustment									(48,113,000)	5,544,000	18,449,000
Net reserves at end of period	2,657,622,000				2,779,401,000				2,657,622,000	2,779,401,000	2,749,803,000
Plus reinsurance recoverables on reserves	1,070,463,000				1,122,731,000				1,070,463,000	1,122,731,000	1,018,047,000
Reserves at end of period	$ 3,728,085,000				$ 3,902,132,000				$ 3,728,085,000	$ 3,902,132,000	$ 3,767,850,000