Fair Value Measurements - Change in Balance Sheet Carrying Value Associated with Level 3 Financial Instruments Carried at Fair Value on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Fair Value Disclosures [Abstract]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	$ 0	$ (21,490)	$ 23,149
Change in balance sheet carrying value associated with Level 3 financial instruments carried at fair value on a recurring basis [Roll Forward]
Balance, beginning of period	0	532,155	498,657
Total gains/(losses) (realized and unrealized):
Included in other comprehensive income	0	63,813	0
Included in earnings - accretion of discount	0	7,596	10,349
Proceeds from sale	0	(582,074)	0
Transfers in and/or out of level 3	0	0	0
Balance, end of period	0	0	532,155
Change in unrealized gains/(losses) relating to instruments still held at the reporting date	$ 0	$ 0	$ 23,149