Condensed Consolidated Balance Sheets (Parenthetical) $ in Thousands	Jun. 30, 2019 USD ($) shares	Jun. 30, 2019 € / shares	Dec. 31, 2018 USD ($) shares	Dec. 31, 2018 € / shares
Current assets:
Accounts receivable, allowances | $	$ 17,726		$ 13,762
Accumulated amortization | $	$ 1,968,741		$ 1,896,861
Shareholders’ equity:
Ordinary shares, nominal value per share (in euros per share) | € / shares		€ 0.01		€ 0.01
Ordinary shares, shares authorized (in shares)	177,069,000		177,069,000
Ordinary shares, shares issued (in shares)	172,325,000		171,719,000
Treasury stock, shares (in shares)	10,986,000		7,571,000