Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income - Detailed Activities of Equity (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Schedule Of Stockholders Equity [Line Items]
Balance			$ 1,308,498
Balance, shares			46,147,788
Common stock issuances			5,539
Net income (loss)	10,067	(3,888)	90,741	74,947	148,433	114,343
Other comprehensive income (loss):
Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax	650	476	1,301	952	(4,636)	(7,826)
Amounts reclassified into net income	519	519	1,037	700	2,074	1,062
Dividends declared Common: $0.66 per share			(30,950)
Balance	1,376,166		1,376,166		1,376,166
Balance, shares	46,334,769		46,334,769		46,334,769
Common Stock [Member]
Schedule Of Stockholders Equity [Line Items]
Balance			47,778
Balance, shares			46,148
Common stock issuances			187
Common stock issuances, shares			187
Other comprehensive income (loss):
Balance	47,965		47,965		47,965
Balance, shares	46,335		46,335		46,335
Additional Paid-in Capital [Member]
Schedule Of Stockholders Equity [Line Items]
Balance			828,777
Common stock issuances			5,352
Other comprehensive income (loss):
Balance	834,129		834,129		834,129
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			1,301
Amounts reclassified into net income			1,037
Balance	(48,407)		(48,407)		(48,407)
Retained Earnings [Member]
Schedule Of Stockholders Equity [Line Items]
Balance			484,369
Net income (loss)			90,881
Other comprehensive income (loss):
Dividends declared Common: $0.66 per share			(30,950)
Balance	544,300		544,300		544,300
Non-controlling Interest [Member]
Schedule Of Stockholders Equity [Line Items]
Balance			(1,681)
Net income (loss)			(140)
Other comprehensive income (loss):
Balance	$ (1,821)		$ (1,821)		$ (1,821)