Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income - Detailed Activities of Equity and Redeemable Noncontrolling Interest (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Schedule Of Stockholders Equity [Line Items]
Balance			$ 1,486,266
Balance			$ 20,042
Balance, shares			46,523,184
Common stock issuances			$ 38,319
Net income (loss)	$ (4,734)	$ 1,970	72,198	$ 82,380	$ 130,944	$ 139,683
Net income (loss)			72,278
Redemption value adjustments			6,738
Balance	13,703		13,703		13,703
Foreign currency exchange translation adj.			(1,518)
Other comprehensive income (loss):
Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax	708	512	2,124	1,535	(9,324)	6,556
Amounts reclassified into net income	519	$ 518	1,556	$ 1,555	2,074	$ 2,074
Dividends declared Common: $1.215 per share			(57,831)
Balance	$ 1,547,932		$ 1,547,932		$ 1,547,932
Balance, shares	47,375,398		47,375,398		47,375,398
Common Stock [Member]
Schedule Of Stockholders Equity [Line Items]
Balance			$ 48,153
Balance, shares			46,523,000
Common stock issuances			$ 852
Common stock issuance, shares			852,000
Other comprehensive income (loss):
Balance	$ 49,005		$ 49,005		$ 49,005
Balance, shares	47,375,000		47,375,000		47,375,000
Additional Paid-in Capital [Member]
Schedule Of Stockholders Equity [Line Items]
Balance			$ 851,381
Common stock issuances			37,467
Redemption value adjustments			5,777
Other comprehensive income (loss):
Balance	$ 894,625		894,625		$ 894,625
Accumulated Other Comprehensive Income (Loss) [Member]
Schedule Of Stockholders Equity [Line Items]
Balance			(50,175)
Foreign currency exchange translation adj.			(1,518)
Other comprehensive income (loss):
Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax			2,124
Amounts reclassified into net income			1,556
Balance	(48,013)		(48,013)		(48,013)
Retained Earnings [Member]
Schedule Of Stockholders Equity [Line Items]
Balance			639,164
Net income (loss)			72,198
Redemption value adjustments			961
Other comprehensive income (loss):
Dividends declared Common: $1.215 per share			(57,831)
Balance	654,492		654,492		654,492
Non-controlling Interest [Member]
Schedule Of Stockholders Equity [Line Items]
Balance			(2,257)
Net income (loss)			80
Other comprehensive income (loss):
Balance	(2,177)		(2,177)		(2,177)
Redeemable Noncontrolling Interest (Temporary Equity) [Member]
Schedule Of Stockholders Equity [Line Items]
Balance			20,042
Net income (loss)			454
Redemption value adjustments			(6,738)
Foreign currency exchange translation adj.			(55)
Balance	$ 13,703		$ 13,703		$ 13,703