Regulatory Matters - Schedule of Capital Ratios (Detail) - USD ($)	Dec. 31, 2015	Dec. 31, 2014
F.N.B. Corporation [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital, Actual Amount	$ 1,629,270,000	$ 1,417,369,000
Tier 1 Capital, Actual Amount	1,321,972,000	1,269,033,000
Common equity tier 1, Actual Amount	1,200,715,000
Leverage, Actual Amount	$ 1,321,972,000	$ 1,269,033,000
Total Capital, Actual Ratio	12.90%	12.40%
Tier 1 Capital, Actual Ratio	10.40%	11.10%
Common equity tier 1, Actual Ratio	9.50%
Leverage, Actual Ratio	8.10%	8.40%
Total Capital, Well-Capitalized Requirements Amount	$ 1,268,396,000	$ 1,146,556,000
Tier 1 Capital, Well-Capitalized Requirements Amount	1,014,717,000	687,934,000
Common equity tier 1, Well-Capitalized Requirements Amount	824,457,000
Leverage, Well-Capitalized Requirements Amount	$ 811,553,000	$ 752,593,000
Total capital, Well-Capitalized Requirements Ratio	10.00%	10.00%
Tier 1 capital, Well-Capitalized Requirements Ratio	8.00%	6.00%
Common equity tier 1, Well-Capitalized Requirements Ratio	6.50%
Leverage, Well-Capitalized Requirements Ratio	5.00%	5.00%
Total capital, Minimum Capital Requirements Amount	$ 1,014,717,000	$ 917,245,000
Tier 1 capital, Minimum Capital Requirements Amount	761,038,000	458,623,000
Common equity tier 1, Minimum Capital Requirements Amount	570,778,000
Leverage, Minimum Capital Requirements Amount	$ 649,243,000	$ 602,074,000
Total capital, Minimum Capital Requirements Ratio	8.00%	8.00%
Tier 1 capital, Minimum Capital Requirements Ratio	6.00%	4.00%
Common equity tier 1, Minimum Capital Requirements Ratio	4.50%
Leverage, Minimum Capital Requirements Ratio	4.00%	4.00%
FNBPA [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital, Actual Amount	$ 1,426,284,000	$ 1,321,433,000
Tier 1 Capital, Actual Amount	1,289,965,000	1,200,776,000
Common equity tier 1, Actual Amount	1,209,965,000
Leverage, Actual Amount	$ 1,289,965,000	$ 1,200,776,000
Total Capital, Actual Ratio	11.30%	11.50%
Tier 1 Capital, Actual Ratio	10.20%	10.50%
Common equity tier 1, Actual Ratio	9.60%
Leverage, Actual Ratio	8.00%	8.10%
Total Capital, Well-Capitalized Requirements Amount	$ 1,265,990,000	$ 1,147,427,000
Tier 1 Capital, Well-Capitalized Requirements Amount	1,012,792,000	688,456,000
Common equity tier 1, Well-Capitalized Requirements Amount	822,893,000
Leverage, Well-Capitalized Requirements Amount	$ 803,041,000	$ 744,235,000
Total capital, Well-Capitalized Requirements Ratio	10.00%	10.00%
Tier 1 capital, Well-Capitalized Requirements Ratio	8.00%	6.00%
Common equity tier 1, Well-Capitalized Requirements Ratio	6.50%
Leverage, Well-Capitalized Requirements Ratio	5.00%	5.00%
Total capital, Minimum Capital Requirements Amount	$ 1,012,792,000	$ 917,941,000
Tier 1 capital, Minimum Capital Requirements Amount	759,594,000	458,971,000
Common equity tier 1, Minimum Capital Requirements Amount	569,695,000
Leverage, Minimum Capital Requirements Amount	$ 642,433,000	$ 595,388,000
Total capital, Minimum Capital Requirements Ratio	8.00%	8.00%
Tier 1 capital, Minimum Capital Requirements Ratio	6.00%	4.00%
Common equity tier 1, Minimum Capital Requirements Ratio	4.50%
Leverage, Minimum Capital Requirements Ratio	4.00%	4.00%