Product warranties (Provision for warranty obligations) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Product Warranties Disclosures [Abstract]
Product warranty period, maximum	50 years
Product Warranty Accrual [Roll Forward]
Balance at beginning of period	$ 30,144	$ 37,265	$ 66,545
Warranty claims paid during the period	(55,314)	(57,163)	(77,017)
Pre-existing warranty accrual adjustment during the period	0	4,473	2,261
Warranty expense during the period	58,100	45,569	45,476
Balance at end of period	$ 32,930	$ 30,144	$ 37,265