Supplemental balance sheet information (intangibles) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013
Intangibles [Line Items]
Gross carrying amount	$ 523,717		$ 523,717		$ 499,662
Accumulated amortization	(369,277)		(369,277)		(346,086)
Intangibles, net	154,440		154,440		153,576
Estimated future amortization expense [Abstract]
Remainder of 2014	21,167		21,167
2015	35,898		35,898
2016	23,845		23,845
2017	11,791		11,791
2018	8,043		8,043
Amortization of intangibles	12,091	11,655	24,115	22,998
Indefinite-lived intangibles [Member] | Trade names [Member]
Intangibles [Line Items]
Gross carrying amount	19,100		19,100		19,100
Intangibles, net	19,100		19,100		19,100
Amortizable intangibles [Member]
Intangibles [Line Items]
Gross carrying amount	504,617		504,617		480,562
Accumulated amortization	(369,277)		(369,277)		(346,086)
Intangibles, net	135,340		135,340		134,476
Amount acquired			26,276
Weighted-average amortization period (in years)			5 years
Amortizable intangibles [Member] | Internal-use software [Member]
Intangibles [Line Items]
Gross carrying amount	359,701		359,701		339,995
Accumulated amortization	(292,485)		(292,485)		(275,159)
Intangibles, net	67,216		67,216		64,836
Amount acquired			19,704
Weighted-average amortization period (in years)			4 years
Amortizable intangibles [Member] | Customer lists/relationships [Member]
Intangibles [Line Items]
Gross carrying amount	68,995		68,995		63,282
Accumulated amortization	(34,989)		(34,989)		(31,606)
Intangibles, net	34,006		34,006		31,676
Amount acquired			6,522
Weighted-average amortization period (in years)			8 years
Amortizable intangibles [Member] | Trade names [Member]
Intangibles [Line Items]
Gross carrying amount	67,761		67,761		67,961
Accumulated amortization	(35,648)		(35,648)		(33,642)
Intangibles, net	32,113		32,113		34,319
Amortizable intangibles [Member] | Other [Member]
Intangibles [Line Items]
Gross carrying amount	8,160		8,160		9,324
Accumulated amortization	(6,155)		(6,155)		(5,679)
Intangibles, net	2,005		2,005		3,645
Amount acquired			$ 50
Weighted-average amortization period (in years)			2 years