SHAREHOLDERS' EQUITY AND SHARE-BASED COMPENSATION - Market-based options vesting prices (Details)	Dec. 31, 2017 $ / shares shares
Ordinary performance grants
Market prices for market performance options to vest
Market-based options, options expected to vest (in shares)	155,023
Market-based options, weighted average share price (in dollars per share) | $ / shares	$ 45.99
Extraordinary performance grants
Market prices for market performance options to vest
Market-based options, options expected to vest (in shares)	111,599
Market-based options, weighted average share price (in dollars per share) | $ / shares	$ 45.83
$40.01 — $50.00
Market prices for market performance options to vest
Market-based options, vesting price, low end of range (in dollars per share) | $ / shares	40.01
Market-based options, vesting price, high end of range (in dollars per share) | $ / shares	$ 50.00
$40.01 — $50.00 | Ordinary performance grants
Market prices for market performance options to vest
Market-based options, options expected to vest (in shares)	9,525
$40.01 — $50.00 | Extraordinary performance grants
Market prices for market performance options to vest
Market-based options, options expected to vest (in shares)	0
$50.01 — $60.00
Market prices for market performance options to vest
Market-based options, vesting price, low end of range (in dollars per share) | $ / shares	$ 50.01
Market-based options, vesting price, high end of range (in dollars per share) | $ / shares	$ 60.00
$50.01 — $60.00 | Ordinary performance grants
Market prices for market performance options to vest
Market-based options, options expected to vest (in shares)	82,600
$50.01 — $60.00 | Extraordinary performance grants
Market prices for market performance options to vest
Market-based options, options expected to vest (in shares)	11,653
$60.01 — $70.00
Market prices for market performance options to vest
Market-based options, vesting price, low end of range (in dollars per share) | $ / shares	$ 60.01
Market-based options, vesting price, high end of range (in dollars per share) | $ / shares	$ 70.00
$60.01 — $70.00 | Ordinary performance grants
Market prices for market performance options to vest
Market-based options, options expected to vest (in shares)	14,148
$60.01 — $70.00 | Extraordinary performance grants
Market prices for market performance options to vest
Market-based options, options expected to vest (in shares)	6,325
$70.01 — $80.00
Market prices for market performance options to vest
Market-based options, vesting price, low end of range (in dollars per share) | $ / shares	$ 70.01
Market-based options, vesting price, high end of range (in dollars per share) | $ / shares	$ 80.00
$70.01 — $80.00 | Ordinary performance grants
Market prices for market performance options to vest
Market-based options, options expected to vest (in shares)	1,250
$70.01 — $80.00 | Extraordinary performance grants
Market prices for market performance options to vest
Market-based options, options expected to vest (in shares)	10,333
$80.01 — $90.00
Market prices for market performance options to vest
Market-based options, vesting price, low end of range (in dollars per share) | $ / shares	$ 80.01
Market-based options, vesting price, high end of range (in dollars per share) | $ / shares	$ 90.00
$80.01 — $90.00 | Ordinary performance grants
Market prices for market performance options to vest
Market-based options, options expected to vest (in shares)	0
$80.01 — $90.00 | Extraordinary performance grants
Market prices for market performance options to vest
Market-based options, options expected to vest (in shares)	30,963
$90.01 — $100.00
Market prices for market performance options to vest
Market-based options, vesting price, low end of range (in dollars per share) | $ / shares	$ 90.01
Market-based options, vesting price, high end of range (in dollars per share) | $ / shares	$ 100.00
$90.01 — $100.00 | Ordinary performance grants
Market prices for market performance options to vest
Market-based options, options expected to vest (in shares)	0
$90.01 — $100.00 | Extraordinary performance grants
Market prices for market performance options to vest
Market-based options, options expected to vest (in shares)	7,075
$110.01 — $120.00
Market prices for market performance options to vest
Market-based options, vesting price, low end of range (in dollars per share) | $ / shares	$ 110.01
Market-based options, vesting price, high end of range (in dollars per share) | $ / shares	$ 120.00
$110.01 — $120.00 | Ordinary performance grants
Market prices for market performance options to vest
Market-based options, options expected to vest (in shares)	0
$110.01 — $120.00 | Extraordinary performance grants
Market prices for market performance options to vest
Market-based options, options expected to vest (in shares)	625
$140.01 — $150.00
Market prices for market performance options to vest
Market-based options, vesting price, low end of range (in dollars per share) | $ / shares	$ 140.01
Market-based options, vesting price, high end of range (in dollars per share) | $ / shares	$ 150.00
$140.01 — $150.00 | Ordinary performance grants
Market prices for market performance options to vest
Market-based options, options expected to vest (in shares)	12,500
$140.01 — $150.00 | Extraordinary performance grants
Market prices for market performance options to vest
Market-based options, options expected to vest (in shares)	0
$170.01 — $180.00
Market prices for market performance options to vest
Market-based options, vesting price, low end of range (in dollars per share) | $ / shares	$ 170.01
Market-based options, vesting price, high end of range (in dollars per share) | $ / shares	$ 180.00
$170.01 — $180.00 | Ordinary performance grants
Market prices for market performance options to vest
Market-based options, options expected to vest (in shares)	12,500
$170.01 — $180.00 | Extraordinary performance grants
Market prices for market performance options to vest
Market-based options, options expected to vest (in shares)	0
$180.01 — $190.00
Market prices for market performance options to vest
Market-based options, vesting price, low end of range (in dollars per share) | $ / shares	$ 180.01
Market-based options, vesting price, high end of range (in dollars per share) | $ / shares	$ 190.00
$180.01 — $190.00 | Ordinary performance grants
Market prices for market performance options to vest
Market-based options, options expected to vest (in shares)	7,500
$180.01 — $190.00 | Extraordinary performance grants
Market prices for market performance options to vest
Market-based options, options expected to vest (in shares)	19,625
Over $190.00 | Ordinary performance grants
Market prices for market performance options to vest
Market-based options, options expected to vest (in shares)	15,000
Over $190.00 | Extraordinary performance grants
Market prices for market performance options to vest
Market-based options, options expected to vest (in shares)	25,000