Business Segments - Financial Information for Segments of FNB (Detail) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Segment Reporting Information [Line Items]
Interest income		$ 678,963	$ 546,795	$ 508,983
Interest expense		67,451	48,573	42,686
Net interest income		611,512	498,222	466,297
Provision for credit losses		55,752	40,441	38,648
Non-interest income		201,761	162,410	158,274
Non-interest expense	[1]	499,923	382,244	369,536
Amortization of intangibles		11,210	8,305	9,717
Income taxes (benefit)		75,497	69,993	62,620
Net income (loss)		170,891	159,649	144,050
Total assets		21,844,817	17,557,662	16,127,090
Total intangibles		1,099,456	878,730	879,717
Operating Segments [Member] | Community Banking [Member]
Segment Reporting Information [Line Items]
Interest income		629,700	500,030	463,376
Interest expense		56,182	41,227	36,318
Net interest income		573,518	458,803	427,058
Provision for credit losses		47,386	32,125	30,872
Non-interest income		151,149	116,141	115,858
Non-interest expense	[1]	435,089	319,923	311,834
Amortization of intangibles		10,526	7,544	9,025
Income taxes (benefit)		70,756	65,071	57,634
Net income (loss)		160,910	150,281	133,551
Total assets		21,659,634	17,374,384	15,944,040
Total intangibles		1,075,118	853,551	857,066
Operating Segments [Member] | Wealth Management [Member]
Segment Reporting Information [Line Items]
Non-interest income		35,283	35,246	31,497
Non-interest expense	[1]	27,201	27,264	25,338
Amortization of intangibles		259	273	288
Income taxes (benefit)		2,845	2,803	2,135
Net income (loss)		4,978	4,906	3,736
Total assets		19,619	20,753	20,877
Total intangibles		10,189	10,447	10,720
Operating Segments [Member] | Insurance [Member]
Segment Reporting Information [Line Items]
Interest income		84	89	98
Net interest income		84	89	98
Non-interest income		14,750	13,052	13,598
Non-interest expense	[1]	12,965	13,891	11,558
Amortization of intangibles		425	488	404
Income taxes (benefit)		532	(412)	625
Net income (loss)		912	(826)	1,109
Total assets		22,053	22,207	19,222
Total intangibles		12,340	12,923	10,122
Operating Segments [Member] | Consumer Finance [Member]
Segment Reporting Information [Line Items]
Interest income		40,922	39,868	38,914
Interest expense		3,759	3,518	3,352
Net interest income		37,163	36,350	35,562
Provision for credit losses		6,706	7,396	6,920
Non-interest income		3,002	2,926	2,919
Non-interest expense	[1]	21,662	20,189	19,692
Income taxes (benefit)		4,488	4,709	4,430
Net income (loss)		7,309	6,982	7,439
Total assets		193,349	195,048	187,796
Total intangibles		1,809	1,809	1,809
Parent and Other [Member]
Segment Reporting Information [Line Items]
Interest income		8,257	6,808	6,595
Interest expense		7,510	3,828	3,016
Net interest income		747	2,980	3,579
Provision for credit losses		1,660	920	856
Non-interest income		(2,423)	(4,955)	(5,598)
Non-interest expense	[1]	3,006	977	1,114
Income taxes (benefit)		(3,124)	(2,178)	(2,204)
Net income (loss)		(3,218)	(1,694)	(1,785)
Total assets		$ (49,838)	$ (54,730)	$ (44,845)

[1] Excludes amortization of intangibles, which is presented separately.