Consolidated Statement of Comprehensive Income (USD $)
In Thousands, unless otherwise specified
 3 Months Ended 9 Months Ended
Sep. 30, 2014
Sep. 30, 2013
Sep. 30, 2014
Sep. 30, 2013
Net income $ 33,154 $ 22,191 $ 69,774 $ 56,860
Other comprehensive income, net of tax:        
Amortization of prior service credit included in net periodic cost, net of tax of $(867) and $(870), $(2,601)and $(2,611) (1,357) [1] (1,362) [1] (4,069) [1] (4,085) [1]
Amortization of actuarial loss included in net periodic cost, net of tax of $1,621 and $2,255, $4,866 and $6,768 2,538 [1] 3,529 [1] 7,612 [1] 10,586 [1]
Other comprehensive income, net of tax 1,181 [1] 2,167 [1] 3,543 [1] 6,501 [1]
Comprehensive income $ 34,335 $ 24,358 $ 73,317 [1] $ 63,361
[1] (1) Amortization of prior service credit (cost) and amortization of actuarial loss are included in the computation of net periodic cost.