Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income - Details of Activities in Equity (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Schedule Of Stockholders Equity [Line Items]
Balance			$ 1,308,498
Balance, shares			46,147,788
Common stock issuances			7,390
Net income (loss)	(3,057)	(4,414)	87,684	70,533	149,790	125,676
Other comprehensive income (loss):
Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax	651	476	1,952	1,428	(4,461)	(7,670)
Amounts reclassified into net income	518	518	1,555	1,218	2,074	1,399
Dividends declared Common: $0.99 per share			(46,415)
Balance	1,360,664		1,360,664		1,360,664
Balance, shares	46,340,002		46,340,002		46,340,002
Common Stock [Member]
Schedule Of Stockholders Equity [Line Items]
Balance			47,778
Balance, shares			46,148
Common stock issuances			192
Common stock issuances, shares			192
Other comprehensive income (loss):
Balance	47,970		47,970		47,970
Balance, shares	46,340		46,340		46,340
Additional Paid-in Capital [Member]
Schedule Of Stockholders Equity [Line Items]
Balance			828,777
Common stock issuances			7,198
Other comprehensive income (loss):
Balance	835,975		835,975		835,975
Accumulated Other Comprehensive Income (Loss) [Member]
Schedule Of Stockholders Equity [Line Items]
Balance			(50,745)
Other comprehensive income (loss):
Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax			1,952
Amounts reclassified into net income			1,555
Balance	(47,238)		(47,238)		(47,238)
Retained Earnings [Member]
Schedule Of Stockholders Equity [Line Items]
Balance			484,369
Net income (loss)			88,017
Other comprehensive income (loss):
Dividends declared Common: $0.99 per share			(46,415)
Balance	525,971		525,971		525,971
Non-controlling Interest [Member]
Schedule Of Stockholders Equity [Line Items]
Balance			(1,681)
Net income (loss)			(333)
Other comprehensive income (loss):
Balance	$ (2,014)		$ (2,014)		$ (2,014)