Securities - Roll-Forward of Other-Than-Temporary Impairment Amount Related to Credit Losses (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Marketable Securities [Abstract]
Balance of credit losses on debt securities for which a portion of other-than-temporary impairment was recognized in earnings, beginning of period	$ 4,054	$ 4,054	$ 4,054	$ 4,054
Additional credit loss for which other-than-temporary impairment was not previously recognized	0	0	0	0
Additional credit loss for which other-than-temporary impairment was previously recognized	0	0	0	0
Balance of credit losses on debt securities for which a portion of other-than-temporary impairment was recognized in earnings, end of period	$ 4,054	$ 4,054	$ 4,054	$ 4,054