Consolidated Statements of Comprehensive Income - Parenthetical (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Net unrealized loss on derivative instruments arising during the period, tax	$ (3,168)	$ (2,225)
Less: Reclassification adjustment for gain on derivative instruments included in net income, tax	1,626	1,366
Net unrealized gains on available-for-sale investments arising during the period, tax	(24)	(2)
Less: Reclassification adjustment for loss on available-for-sale investments included in net income, tax	(19)	(16)
Amortization of postretirement liability losses included in net periodic benefit cost, tax	319	0
Foreign currency translation adjustment, tax	$ 37	$ 138