Stockholders' equity Accumulated other comprehensive loss (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Accumulated Other Comprehensive Income (Loss) [Line Items]
Unrealized holding gains (losses) arising during the period	$ 15,989	$ 55,401	$ (31,434)	$ 83,836
Increase (Decrease) Accumulated Other Comprehensive Income (Loss) Net of Tax [Roll Forward]
Balance as of December 31, 2015			49,191
Other comprehensive (loss) income before reclassifications			(39,907)
Amounts reclassified from other comprehensive (income) loss			(1,036)
Total other comprehensive income (loss)	16,781	54,741	(40,943)	84,736
Other Comprehensive Income (loss), Net of Tax, Attributablee to Noncontrolling Interests			24
Net other comprehensive (loss) income			(40,919)
Balance as of June 30, 2016	8,272		8,272
Unrealized interest rate hedge losses arising during the period	(3,676)	(1,225)	(10,637)	(4,238)
Unrealized foreign currency translation (losses) gains arising during the period	(1,364)	(1,507)	2,164	(7,778)
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax	3,061	(1,362)	10,087	(2,465)
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax	(1,865)	(710)	(2,023)	(1,215)
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Reclassification Adjustment from AOCI, Realized upon Sale or Liquidation, Net of Tax	(7,028)	$ 0	(7,028)	$ (9,236)
Available-for- Sale Equity Securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Unrealized holding gains (losses) arising during the period			(31,434)
Increase (Decrease) Accumulated Other Comprehensive Income (Loss) Net of Tax [Roll Forward]
Balance as of December 31, 2015			117,599
Total other comprehensive income (loss)			(41,521)
Other Comprehensive Income (loss), Net of Tax, Attributablee to Noncontrolling Interests			0
Net other comprehensive (loss) income			(41,521)
Balance as of June 30, 2016	76,078		76,078
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax			10,087
Interest Rate Hedge Agreements
Increase (Decrease) Accumulated Other Comprehensive Income (Loss) Net of Tax [Roll Forward]
Balance as of December 31, 2015			(3,718)
Total other comprehensive income (loss)			(8,614)
Other Comprehensive Income (loss), Net of Tax, Attributablee to Noncontrolling Interests			0
Net other comprehensive (loss) income			(8,614)
Balance as of June 30, 2016	(12,332)		(12,332)
Unrealized interest rate hedge losses arising during the period			(10,637)
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax			(2,023)
Foreign Currency Translation
Increase (Decrease) Accumulated Other Comprehensive Income (Loss) Net of Tax [Roll Forward]
Balance as of December 31, 2015			(64,690)
Total other comprehensive income (loss)			9,192
Other Comprehensive Income (loss), Net of Tax, Attributablee to Noncontrolling Interests			24
Net other comprehensive (loss) income			9,216
Balance as of June 30, 2016	$ (55,474)		(55,474)
Unrealized foreign currency translation (losses) gains arising during the period			2,164
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Reclassification Adjustment from AOCI, Realized upon Sale or Liquidation, Net of Tax			$ (7,028)