Securities - Additional Information (Detail) (USD $)	3 Months Ended		12 Months Ended
	Mar. 31, 2015 Holdings	Mar. 31, 2014	Dec. 31, 2014 Holdings	Dec. 31, 2013
Amortized Cost and Fair Value Debt Securities [Abstract]
Maximum percentage of equity of one issuer	10.00%
Number of holdings greater than specified percentage of equity	0		0
Securities with aggregate fair values	$ 1,100,000,000		$ 706,500,000
Proceeds from sale of available-for-sale securities	560,676,000	3,155,000	3,200,000
Net unrealized gains (losses) on available-for-sale securities included in AOCI	7,685,000	(3,002,000)	2,892,000	(6,126,000)
Impaired loss relating to securities	0
Federal home loan bank stock, Total	$ 20,900,000		$ 11,600,000